bluuebunny/crossref_metadata_2025 huggingface dataset:
Multilayer Lead Zirconate Titanate and Barium Titanate Ferroelectric Capacitors
Authors: Chang L.H. , Anderson W.A.
Published: April 2011

Ferroelectric PbZrxT1−xO3 (PZT) thin films have been deposited on Pt coated Si substrates by if magnetron sputtering. The optimized processing condition to obtain proper stoichiometric PZT, the desired ferroelectric perovskite phase, and better dielectric properties was demonstrated using a PZT target with Pb(Zr+Ti) ratio of 1.2 and depositing at 350°C, followed by thermal treatment at 620°C for 30 min. The structural and electrical properties of the PZT layer were further improved by fabricating a novel multilayer structure which combined the PZT with the nanolayer BaTiO3. The leakage current density was reduced from 2×10−7 A/cm2 for the single layer structure to 2×10−9 A/cm2 for the multilayer structure at a field of 4×105 V/cm, while maintaining a high relative effective dielectric constant of 442. The relative dielectric constant of the PZT film in the multilayer structure was calculated to be about 880.
Influence of Strontium Concentration on the Microstructure and Electrical Properties of Sol-Gel Derived Barium Strontium Titanate Thin Films
Authors: Gust M.C. , Momoda L.A. , Mecartney M.L.
Published: April 2011

BaxSrl−xTiO3 thin films with varying Sr concentration were prepared on Pt coated Si substrates using methoxypropoxide based alkoxide precursors. Films were crystallized by heat treating at 700°C for 30 minutes in an oxygen atmosphere after deposition of each layer. Film thickness ranged from 230 to 260 nm. No evidence of tetragonality was observed in any of the compositions. Films with higher Sr concentrations had a larger average grain size, larger grain size distribution, and increased (111) orientation on (111) oriented Pt. The highest dielectric constant of ˜400 was found for Ba 0.5Sr0.5TiO3, although no direct correlation could be made between the composition and dielectric properties.
Influence of PbTiO3 Buffer Layers on Microstructural Properties of Pb(Zr,Ti)O3 Films Deposited by Sputtering
Authors: Cattan E. , Roma R. , Velu G. , Jaber B. , Remiens D. , Thierry B.
Published: April 2011

Pb(Zr,Ti)O3 (PZT) films with very thin PbTiO3 (PT) buffer layers inserted between the film and the Si/SiO2/Ti/Pt substrate were grown by a rf magnetron sputtering technique. The Xray diffraction patterns and the surface morphology of PT as a function of the thickness, in the range of 20 to 400 Å, were first investigated. Such a marked contrast in morphological features was not observed when the thickness was above 400 Å. The films were grown in-situ at 500°C. We found modification of the PT orientation when its thickness varied between 20 Å and 400 Å. This implies a gradual evolution of the lattice parameters in the nucleation stage of PT films. Insertion of these buffer layers was found to exert marked effects. Up to 400 Å the PT was grown with a [100] preferred orientation. The orientation of PZT could be controlled by changing the thickness and then the orientation of PT. The results will be discussed in terms of the effects of the buffer layer on the crystalline structure and the surface morphology of PZ
Characterization of Mis Capacitor of Bst Thin Films Deposited on Si by Rf Magnetron Sputtering
Authors: Paek S.H. , Park C.S. , Won J.H. , Lee K.S.
Published: April 2011

The application of high dielectric (Ba, Sr)TiO3 [BST] thin films for Metal-Insulator- Semiconductor(MIS) capacitors was investigated. BST thin films were deposited on p-Si(100) substrates by the RF magnetron sputtering with temperature range of 500–600 °C. We examined the characteristics of MIS capacitor with various oxygen pressure, substrate temperature and (Ba+Sr)/Ti ratio. The dielectric properties of MIS capacitors consisting of AI/BST/SiO2/Si sandwich structure were measured for various conditions. The charge state densities of the MIS capacitors were determined by high frequency (1 MHz) C-V measurement. Also, current-voltage characteristics of the MIS capacitor were investigated. In order to reduce the leakage current in MIS capacitor, high quality SiO2 layer was grown on bare p-Si substrate by thermal oxidation. By applying SiO2 layer between BST thin films and Si substrate, low leakage current of 10−10 order was observed. Futhermore, the leakage current showed the dependence on the oxygen concentration in plasma gas and the (Ba+Sr)/Ti ratio. Also, the BST MIS structure showed relatively high capacitance even though it is the combination of high-dielectric BST thin films and SiO2 layer. By C-V measurement, the polarity of effective oxide charge changed with the oxygen concentration in plasma gas and (Ba+Sr)/Ti ratio of sputtering target.
Synthesis and Mechanical Properties of Niobium Films by Ion Beam Assisted Deposition
Authors: Ji H. , Was G. S. , Jones J. W.
Published: April 2011

Mechanical properties of niobium thin films are studied by controlling the microstructure, texture and residual stress of the films using ion beam assisted deposition (IBAD). Niobium films were deposited onto (100) Si substrates and their microstructure, texture and residual stress were measured as a function of ion energy and R ratio (ion to atom arrival rate ratio). The grain sizes of these films ranged from 20 nm to 40 nm and no effect of ion bombardment was observed. All the films have strong (110) fiber texture, but the in-plane texture is a strong function of the incident angle, energy and flux of the ion beam. Results show that while the degree of the texture increases with increasing ion energy and flux, it is also a strong linear function of the product of the two. The residual stress of the films was measured by a scanninglaser reflection technique. As a function of normalized energy, the stress is tensile for En < 30 eV/atom with a maximum of 400 MPa at about 15 eV/atom. It becomes compressive with increasing normalized energy and saturates at - 400 MPa for En > 50 eV/atom. Both PVD (physical vapor deposition) and IBAD films have a hardness of about 6 GPa at shallow depth measured by nanoindentation. The different stress state may be responsible for the 15%difference on hardness observed between the PVD and IBAD films.
Time Dependent Stress Fields Ahead of the Interface Cracks in Creep Regime
Authors: Biner S. B.
Published: April 2011

In this study the behavior of stationary interface cracks in layered materials at creep regime in plane-strain condition and pure opening dominated mode-I load state was studied numerically. The results indicate that the introduction of a transitional layer to the interface of the elastic-creeping bimaterials significantly elevates the stress values ahead of the interface cracks under identically applied load levels at creep regime.
Multi-Phase Functionally Graded Materials for Thermal Barrier Systems
Authors: Jackson M. R. , Ritter A. M. , Gigliotti M. F. , Kaya A. C. , Gallo J. P.
Published: April 2011

Metallic candidates for functionally graded material (FGM) coatings have been evaluated for potential use in bonding zirconia to a single crystal superalloy. Properties for four materials were studied for the low-expansion layer adjacent to the ceramic. Ingots were produced for these materials, and oxidation, expansion and modulus were determined. A finite element model was used to study effects of varying the FGM layers. Elastic modulus dominated stress generation, and a 20–25% reduction in thermal stress generated within the zirconia layer may be possible.
Hardness and Deformation Mechanisms of Highly Elastic Carbon Nitride Thin Films as Studied By Nanoindentation
Authors: Hainsworth S. V. , Sjöström H. , Page T. F. , Sundgren J-E.
Published: April 2011

Carbon nitride (CNx) thin films (0.18<x<0.43), deposited by magnetron sputtering of C in a N2 discharge, have been observed to be extremely resistant to plastic deformation during surface contact (i.e. exhibit a purely elastic response over large strains). Elastic recoveries as high as 90% have been measured by nanoindentation. This paper addresses the problems of estimating Young's modulus (E) and hardness (H) in such cases and shows how different strategies involving analysis of both loading and unloading curves and measuring the work of indentation each present their own problems. The results of some cyclic contact experiments are also presented and possible deformation mechanisms in the fullerene-like CNx structures discussed.
Viscoelastic Properties of Healthy Human Artery Measured in Saline Solution by AFM-Based Indentation Technique
Authors: Lundkvist A. , Lilleodden E. , Siekhaus W. , Kinney J. , Pruitt L. , Balooch M.
Published: April 2011

Using an Atomic Force Microscope with an attachment for indentation, we have measured local, in vitro mechanical properties of healthy femoral artery tissue held in saline solution. The elastic modulus (34.3 kPa) and viscoelastic response (τɛ = 16.9 s and τσ = 29.3 s) of the unstretched, intimal vessel wall have been determined using Sneddon theory and a three element model (standard linear solid) for viscoelastic materials. The procedures necessary to employ the indenting attachment to detect elastic moduli in the kPa range in liquid are described.
Micro-Indentation using Strain Rate Sensitivity to Examine Deformation of Polymeric and Metallic Surface Layers
Authors: Newson W. R. , Diak B. J. , Saimoto S.
Published: April 2011

A sensitive method to characterize the inelastic deformation of solids is to measure the thermodynamic response of the deforming volume by applying an inelastic strain rate change. Using the step ramp technique on a displacement controlled micro-indentor with Berkovich tip, the apparent activation volumes were determined for linear low density polyethylene, and eutectic Pb-Sn solder for depths less than 10 μm. The results show that the activation volume is a unique measure of a given microstructure.
Low Pressure OMVPE Grown Compressive Strained Ingaas QWS Surrounded by Tensile Strained Ingaas Spacers
Authors: Laboutine O. A. , Choo A. G. , Kim S. H. , Kim N. H. , Park H. S. , Park Y. J. , Kim T. I.
Published: April 2011

The photoluminescence (PL) measurement of unstrained and compressive strained InxGa1−xAs quantum wells (QWs) demonstrated that the interfaces had a roughness of 1 to 2.5 monolayers (ML). The spacer layers of tensile strained InyGa1−yAs were inserted between a compressive strained In0.7Ga0.3As QW and the InP barrier layers. PL spectra of the In0.7Ga0.3As/InyGa1−yAs/InP structures grown at 605°C became significantly narrower with increasing Ga content in the spacer layers above (1−y)=0.6. The PL line width of the In0.7Ga0.3As/In0.3Ga0.7As alternatingly strained multiple quantum well (MQW) structures gradually decreased when the number of periods and the deposition temperature increased. The structure, consisting of 4 periods grown at 635°C, exhibited a PL line width of about 4 meV which corresponded to the QW thickness fluctuation of 0.5 ML. The superlattice satellites of X-ray rocking curves of MQW structures were better defined under an envelope corresponding to the tensile strained layers.
Epitaxial Growth and Structure of Highly Mismatched Oxide Films with Rock-salt Structure on MgO
Authors: Langjahr P. A. , Wagner T. , Lange F. F. , Ruole M.
Published: April 2011

CaO thin films were grown on (001) MgO single crystal substrates by a chemical solution deposition method to study the influence of a relatively high lattice mismatch (14%) on the epitaxial growth. In order to minimize an influence of a different crystal structure and chemistry between film and substrate, film and substrate were chosen to have the same structure (NaCl) and a similar chemistry. The films were heat treated until they broke up into single crystal islands on the MgO substrate. X-ray pole figure analysis and TEM observations of the CaO-films showed that most of the grains had a single out-of-plane orientation (111)CaO || (001)MgO, corresponding to two non-redundant, symmetry related in-plane variants [1 10](111)CaO || [110](001)MgO and [110](111)CaO || [110](001)MgO. A near coincidence site lattice (NCSL) model could be constructed for the interface, suggesting good lattice and charge matching for the observed orientation relations. The mechanisms and energetics leading to the experimentally observed orientation relations are discussed.
Effect of Boron Doping on the C49 TO C54 Phase Transformation in Ti/Si (100) Bilayers
Authors: Libera M. , Quintero A.
Published: April 2011

We have demonstrated that the formation of C54 TiSi2 on Boron-doped single crystal silicon substrates, under RTA annealing conditions in a Nitrogen ambient, leads to a thicker TiN capping surface layer, thinner silicide layer, higher C49 to C54 transformation temperature and greater interface roughness compared to C54 TiSi 2 formation on undoped single crystal silicon substrates. Titanium films 32 nm thick were deposited on undoped and boron-doped single crystal silicon substrates. The films were annealed at 3 /C/isn nitrogen to final quenching temperatures between 500 °C and 900 TC. Ex-situ four point probe sheet resistance, cross sectional transmission electron microscopy (XTEM), high resolution transmission electron microscopy (HRTEM) and x-ray diffraction (XRD) were used to analyze the resulting TiN on TiSi2 bilayer. The C49 to C54 transformation occurs circa 760 TC and 810 TC for the undoped and boron-doped cases respectively. HRTEM observations reveal a thick 20 nm TIN layer on the C54 TiSi2 film in the boron-doped case but only fine dispersed TiN particles embedded on the top of the silicide in the undoped case. It was observed that the resultant silicide in the boron-doped case was thinner and the TiSi2 /Si(100) interface is rougher. XRD and TEM analysis show that in the boron doped case, there is a preferred C54 (040) orientation compared to a random orientation for the undoped case.
In-Situ Tem Characterization of Whiskers on Al Electrodes for Thin-Film Transistors
Authors: Tsujimoto K. , Tsuji S. , Saka H. , Kuroda K. , Takatsuji H. , Suzukip Y.
Published: April 2011

The recent attention paid to stress migration of aluminum (Al) electrodes in thin-film transistor liquid crystal display (TFT-LCD) applications indicates that wiring materials with low electrical resistivities are of considerable interest for their potential use in higher-resolution displays. In this paper, we firstly describe how as-grown Al whiskers on Al electrodes fabricated on a LCD-grade glass substrate can be characterized by means of a high-voltage transmission electron microscope (HV-TEM) operated at 1 MV. The whiskers ranging from 300 to 400 nm in diameter are sufficient to be transparent to high-voltage electrons. This allows detailed observation of whisker characteristics such as its morphology and crystallography. In most cases, the as-grown Al whiskers in our study had straight rod shapes, and could be regarded as single crystals. Secondly, we report on the in-situ fabrication and observation of Al whiskers at elevated temperature with the HV-TEM. Since relatively thick TEM samples (up to about 1 mm) can be set on a sample holder in the HV-TEM, various growth stages of Al whiskers can be investigated under various heating conditions. Finally, we demonstrate a TEM sample preparation method for the cross-section of an individual Al whisker, using focused ion beam (FIB) etching. This technique makes it possible to reduce the thickness of an Al whisker close to the root. Both bright- and dark-field TEM images provide nanostructural information on the whisker/Al thin-film interface.
Correlation Between Structural and Electronical Properties Of Strained V2O3 Thin Films
Authors: Schuler H. , Klimm S. , Horn S.
Published: April 2011

Epitaxial thin films of V2O3 were grown on c-axis oriented sapphire substrates by reactive e-beam evaporation. The high orientation and epitaxy is shown by low energy electron and x-ray diffraction. The microstucture of the films was investigated by AFM imaging. The measurements show an island growth mode with a temperature dependent increase of the grain size. At a growth temperature of above 700°C a change of the surface structure is observed, which produces much more structured and irregular shaped crystallites. High temperature growth also causes changes in the electronical properties, due to an increased stress in the films as measured by XRD. The metal-insulator transition (MIT) and the properties of the metallic phase of the films were determined mainly by temperature dependent electrical resistivity measurements. The metallic properties are distorted in films grown at low temperature (T<500°C) probably due to defects, dislocations and grain boundaries. Favorable growth conditions for films showing a single crystal like MIT were found in a narrow temperature range around 550°C. For higher growth temperatures (T>650°C) metallic behavior at high temperatures is suppressed by an in plane tensile stress. The electronic properties of such films are similar to those of chromium doped V2O3. The relation between growth conditions, structural properties and resulting electronic properties is discussed.
Time-resolved RHEED Studies of the Growth of Epitaxial ZnSe Films on GaAs by Pulsed Laser Deposition
Authors: McCamy James W. , Aziz Michael J.
Published: April 2011

Film growth consists of two basic processes, deposition and surface relaxation, with opposing effects on the evolution of surface roughness. The pulsed-laser deposition (PLD) growth process has the unique feature of having periods of very high deposition rates on μs time scales followed by periods, on the order of seconds, with only surface relaxation. In this paper we report the first efforts towards exploiting this unique feature to study these two basic processes independently. Thin epitaxial films of ZnSe were grown using PLD on (001) GaAs and 2° miscut (001) GaAs substrates. For growth on both the singular and vicinal surfaces, RHEED patterns taken following growth showed clear, streaky first zone and sharp second zone spots, and well-defined Kikuchi lines; these features are indicative of a smooth growth surface and high quality film. No reconstruction of the growth surface was observed, in contrast to behavior observed in molecular beam epitaxy. Time-resolved RHEED measurements show that a single morphology developed during growth on singular (001) GaAs. However, during growth on miscut (001) GaAs, two morphologies developed, one transitory and one appearing to evolve towards steady state. When growth on the miscut substrate was stopped, recovery of the RHEED signal was observed. The rate of recovery could be attributed to two relaxation processes, as differentiated by their time constants. Potential origins of these observations are discussed.
Radiochromic Thin-Film Sensor Using Blue Tetrazolium in Polyvinyl Alcohol
Authors: Hsu C.-K. , Al-Sheikhly M. , Mclaughlin W. L. , Christou A.
Published: April 2011

A new radiochromic thin-film sensor using the blue tetrazolium chloride salt (BT) dissolved in polyvinyl alcohol (PVA) has been developed for deep ultraviolet, and gamma and xray dosimetry systems. The radiation-sensitive blue tetrazolium (BT2+), pale yellowish before irradiaton, registers a permanent purple high-resolution image upon irradiation. The distinctive coloration in terms of increase of optical density (net absorbance, ΔA) at 552 nm is measured spectrophotometrically and is proportional to the amount of radiation energy absorbed per unit mass (absorbed dose, D). The films have broad monitoring range, being able to detect radiation doses as low as 1 kGy and as high as 75 kGy. The radiochromic images show a gradual postirradiation increase in absorbance, especially during the first 24 hrs. There is also a small but predictable variation in sensitivity with temperature during irradiation. Furthermore, the responses to gamma radiation are found to be dependent on absorbed dose rate, but can be minimized by increasing the dye precursor concentration (BT2+).
Chemical Etching of AlN and InAlN in KOH Solutions
Authors: Vartuli C. B. , Lee J. W. , MacKenzie J. D. , Pearton S. J. , Abernathy C. R. , Zolper J. C. , Shul R. J. , Ren F.
Published: April 2011

Wet chemical etching of A1N and InxAl1-xN was investigated in KOH-based solutions as a function of etch temperature, and material quality. The etch rates for both materials increased with increasing etch temperatures, which was varied from 20 °C to 80 °C. The crystal quality of A1N prepared by reactive sputtering was improved by rapid thermal annealing at temperatures to 1100 °C with a decreased wet etch rate of the material measured with increasing anneal temperature. The etch rate decreased approximately an order of magnitude at 80 °C etch temperature after a 1100 °C anneal. The etch rate for In0.19Al0.81N grown by Metal Organic Molecular Beam Epitaxy was approximately three times higher for material on Si than on GaAs. This corresponds to the superior crystalline quality of the material grown on GaAs. Etching of InxAl1-xN was also examined as a function of In composition. The etch rate initially increased as the In composition changed from 0 to 36%, and then decreased to 0 Å/min for InN. The activation energy for these etches is very low, 2.0 ± 0.5 kcal•mol-1 for the sputtered A1N. The activation energies for InAIN were dependent on In composition and were in the range 2–6 kcal mol-1. GaN and InN layers did not show any etching in KOH at temperatures up to 80 °C.
Photo-Assisted Anodic Etching of Gallium Nitride Grown by MOCVD
Authors: Lu Hongqiang , Wu Ziming , Bhat Ishwara
Published: April 2011

In this paper, the first study of photo-assisted anodic etching of unintentionally doped n-GaN at room temperature is reported. The electrolyte used is a mixture of buffered aqueous solution of tartaric acid and ethylene glycol. The etching rate varies from ∼20 Å/min to as high as 1600 Å/min. A systematic study shows that i) the etch rate, as well as the surface roughness, increases with the current density; ii) the etching rate is the highest when the pH of the electrolyte is around 7; iii) the etching is faster when there is more ethylene glycol in the electrolyte solution.
Investigation of Aluminum and Titanium/Aluminum Contacts to n-Type Gallium Nitride
Authors: Luther B. P. , Mohney S. E. , Jackson T. N. , Asif Khan M. , Chen Q. , Yang J. W.
Published: April 2011

We report on a study of Al and Ti/Al contacts to n-type GaN. Al contacts on n-GaN (7 x l017cm-3) annealed in forming gas at 600°C reached a minimum contact resistivity of 8x10~6Qcm2 and had much better thermal stability than reported by previous researchers. Ti/Al (35nm/l 15nm) contacts on n-GaN (5xl0I7cm~3) had resistivities of 7 x l0"6Qcm2 and 5xl0"6Qcm2 after annealing in Ar at 400°C for 5min and 600°C for 15sec, respectively. Depth profiles of Ti/Al contacts annealed at 400°C showed that low contact resistance was only achieved after Al diffused to the GaN interface. We propose that the mechanism for ohmic contact formation in Ti/Al contacts annealed in the 400-600°C range includes reduction of the native oxide on GaN by Ti and formation of an Al-Ti intermetallic phase in intimate contact with the GaN. Contacts with different Ti/Al layer thicknesses were investigated and those with 50nm/100nm layers had the same low resistance and better stability than 25nm/125nm contacts.
Optical and Electrical Characteristics of Single-Quantum-Well InGaN Light-Emitting Diodes
Authors: Perlin Piotr , Osiński Marek , Eliseev Petr G
Published: April 2011

We have studied the electroluminescence and photoluminescence of Nichia single-quantum-well Al0.2Ga0.8N/In0.45Ga0.55N/GaN green light-emitting diodes over a broad range of temperatures (15-300 K) and currents (0.2 μA - 2 A). The most striking behavior is an anomalous temperature shift of both photo- and electroluminescence, with the emission peak moving towards higher energies with increasing temperature. This blue shift is opposite to that of the energy gap of the active layer, which practically excludes interband transitions as responsible for the observed optical transitions. We suggest that population effects within the band tails can account for the observed anomaly. We also determined that the current flowing through the p-n junction is dominated by carrier tunneling, the omnipresent effect in the GaN-based optoelectronic devices.
Stimulated Emission from Single- and Multiple-Quantum-Well GaN-AlGaN Separate-Confinement Heterostructures
Authors: Loeber D. A. S. , Anderson N. G. , Redwing J. M. , Flynn J. S. , Smith G. M. , Tischler M. A.
Published: April 2011

Stimulated emission characteristics are examined for GaN-AlGaN separate-confinement quantum-well heterostructures grown by MOVPE on 4H-SiC substrates. We specifically focus on comparison of structures with different quantum well active region designs. Polarization resolved edge emission spectra and stimulated emission thresholds are obtained under optical pumping using a stripe excitation geometry. Stimulated emission characteristics are studied as a function of the number of quantum wells in the structure, and are correlated with surface photoluminescence properties. We find reduced stimulated emission thresholds and increased surface photoluminescence intensities as the number of quantum wells is reduced, with the best results obtained for a single-quantum-well structure. These results should provide useful information for the design of GaN-based quantum well lasers.
III-V Nitride Growth by Atmospheric-Pressure MOVPE with a Three-Layered Flow Channel
Authors: Uchida Kazuo , Tokunaga Hiroki , Inaishi Yoshiaki , Akutsu Nakao , Matsumoto Koh , Itoh Tsuyoshi , Egawa Takashi , Jimbo Takashi , Umeno Masayoshi
Published: April 2011

We introduce III-V nitrides growth including GaN as well as InGaN by a newly developed atmospheric-pressure metal-organic vapor phase epitaxy system with a three layered flow channel which is a promising system for a large scale production. First, we have shown through computer simulation that a laminar flow of gases is maintained at 1000 °C in the three layered flow channel. Second, as a part of epitaxial results, we have found that the surface roughness of a low temperature-grown buffer layer on sapphire substrates, which can be measured by atomic force microscope, should be minimum in order to grow high quality GaN. We also report the growth of a double heterostructure of Ino. 15Gao.85N/GaN which shows a strong near band-edge emission in room temperature photoluminescence.
Kinetics of Nitrogen in GaAsN Layers During GaAs Overgrowth
Authors: Bandić Z. Z. , Hauenstein R. J. , O’Steen M. L. , McGill T. C.
Published: April 2011

A set of GaAsN/GaAs strained layer superlattices was grown on GaAs (001) substrates by electron cyclotron resonance microwave plasma assisted molecular beam epitaxy. An ex-situ high resolution X-ray diffraction was employed to characterize interface quality and determine the effective nitrogen content, as a function of growth temperature, of buried GaAsN layers. The interface quality, which was assessed through FWHM’s and intensities of superlattice peaks, and nitrogen content were found to change dramatically in the temperature range from 540°C to 580°C. A first order kinetic model was introduced to quantitatively explain this dependencies, in terms of energetically favorable N for As anion exchange and thermally activated N desorption and segregation processes. The strong nitrogen surface segregation process, which acts concurrently with nitrogen desorption is found to be responsible for both degradation of interface quality and reducing nitrogen content at elevated temperatures. The predicted nitrogen profile smearing of (2 – 3)nm, obtained from the model, is found to be in good agreement with cross-sectional transmission electron microscopy observations.
Doping Studies of n- and p-Type AlxGa1-xN Grown by ECR-Assisted MBE
Authors: Korakakis D. , Ng H. M. , Ludwig K. F. , Moustakas T. D.
Published: April 2011

AlxGal−xN films (x≤0.60) were grown on c-plane sapphire and (0001) 6H-SiC substrates using ECR plasma assisted Molecular Beam Epitaxy. Evidence of long range ordering in the investigated AlxGal−xN films is presented. Without intentional dopants the films are semi-insulating with resistivities ranging from 103 to 105 Ω.cm. The films were doped n-type with Si and p-type with Mg. The carrier concentration in the Si doped films, as determined by Hall effect measurements, was between 1016 to 1019 cm−3. At constant Si cell temperature, the carrier concentration was found to be reduced with AlN mole fraction, consistent with the observation that the donor ionization energy increases with Al content. Correspondingly, the electron mobility decreases with Al concentration, a result attributed to alloy scattering. The Mg doped films were found to exhibit p-type conductivity by thermoelectric power measurements with resistivities varying from 3 to 30 Ω-cm.
Large-Area Growth of InGaN/AlGaN Using In Situ Monitoring
Authors: Woelk E. , Schmitz D. , Strauch G. , Wachtendorf B. , Jorgensen H.
Published: April 2011

A newly developed Metalorganic Chemical Vapor Deposition (MOCVD) reactor for processing batches of seven 50mm wafers per run at deposition temperatures up to 1600°C is introduced. The substrates are individually rotated by means of gas bearings utilizing high purity gas for operation. In order to achieve the maximum uniformity on a wafer the uniformity of the growth temperature was maximized. Pyrometric measurements revealed a temperature uniformity across the susceptor of ±4°C at a temperature of 1300°C. Growth of GaN and GaInN produced uniform layers regarding composition and thickness. On a 50mm diameter wafer the standard deviation for the thickness of a GaN layer is 6% and the standard deviation for the composition of a GaInN layer as determined by photoluminescence is 2%.
The Growth of GaN Films by Migration-Enhanced Epitaxy
Authors: Hooper S. E. , Foxon C. T. , Cheng T. S. , Jeffs N. J. , Ren G. B. , Lacklison D. E. , Orton J. W. , Duggan G.
Published: April 2011

Gallium Nitride epitaxial films were grown by migration enhanced epitaxy directly on sapphire (0001) without using any pre-growth substrate nitridation or low temperature buffer layers. In comparison with our material grown directly on sapphire by conventional molecular beam epitaxy, a significant improvement in the surface morphology and layer properties, measured by reflection high energy electron diffraction, X-ray diffraction, scanning electron microscopy, room temperature photoluminescence and the Hall effect, was observed for material grown by migration enhanced epitaxy.
Gallium Nitride Thick Layers: Epitaxial Growth and Separation from Substrates
Authors: Bel’kov V. V. , Botnaryuk V. M. , Fedorov L. M. , Diakonu I. I. , Krivolapchyuk V. V. , Scheglov M. P. , Zhilyaev Yu. V.
Published: April 2011

We investigated the possibilities of vapour phase epitaxy in an open tube chloride system for thick GaN film deposition on sapphire substrates. The methodes of the buffer layer deposition were proposed and developed. The methods of fast ( up to 100 microns / hour ) was developed. Parameters of good quality gallium nitride epitaxy were obtained.

To determine the quality of fast grown epitaxial layers we used X-ray diffraction and photoluminescence measurements. The halfwidth of the rocking curve for the best samples was equal to 4-6 minutes. Luminescense spectrum ( T=77K ) had a maximum near 3.46 eV. A signal in the visible wavelength range was hardly observed. Polished layers were transparent.

A special initial treatment of the substrates allowed us to separate thick ( up to 300 micron ) epitaxial gallium nitride layers from sapphire. It was shown that it is possible to use separated films for homoepitaxy of GaN.
Surface Energy Constraints for Heteroepitaxial Growth on Compliant Substrates: Morphology of GaN Grown on Sc Layers
Authors: Koleske D. D. , Wickenden A. E. , Freitas J. A. , Kaplan R. , Prokes S. M.
Published: April 2011

An empirical relationship linking surface energy to bulk modulus is presented which suggests that the thermodynamic growth mode for heteroepitaxy on compliant substrates should be 3-D. As an example of this behavior, GaN growth on Sc is shown for various growth conditions. 2-D growth is obtained when the GaN is grown on top of a low temperature GaN nucleation layer. Our results indicate that surface and interface energies, in addition to, lattice matching and thermal matching play an important role in determining the heteroexpitaxial growth morphology of GaN. There appears to be no net reduction in the dislocation density for GaN films grown on the Sc layers, because the GaN film has to be grown on a low temperature nucleation layer.
ZnO Buffer Formed on Si and Sapphire Substrates for GaN MOVPE
Authors: Shirasawa T. , Honda T. , Koyama F. , Iga K.
Published: April 2011

ZnO layers have been deposited by electron beam (EB) evaporation and laser ablation molecular beam epitaxy (MBE) as buffer layers to grow GaN by metal organic vapor phase epitaxy (MOVPE). The photoluminescence spectrum of the ZnO layer deposited by an EB evaporator shows an emission peaks of 367 nm. GaN was grown on ZnO/Si, Si and sapphire substrates under the same growth condition employing low-temperature-grown AIN buffers to prevent the dissociation of ZnO during the high GaN growth. The GaN on ZnO/Si shows sharp photoluminescence spectra at room temperature and 10 K. These results indicate a potential use of ZnO/Si substrates for GaN based blue-UV optical devices such as vertical-cavity surface-emitting lasers (VCSELs).
MBE Growth and Characterization of Zns/Gan Heterostructures
Authors: Piquette E. C. , Bandić Z. Z. , McCaldin J. O. , McGill T. C.
Published: April 2011

Heterostructures involving ZnS/GaN show promise for the injection of holes from p-GaN into n-ZnS. This combination could result in multi-color electroluminescent displays. We have grown single crystal ZnS on GaN and sapphire (0001) by MBE using elemental sources. The ZnS was grown at temperatures from 150°C–400°C, with beam flux equivalent pressures of (0.3 – 2.0) × 10−7 torr. Growth rates of up to 0.4 μm per hour were observed for the lower growth temperatures, with rapidly diminishing rates for temperatures above 350μC. The GaN substrate consisted of a 3 μm epilayer grown on sapphire by MOCVD. XPS analysis revealed the presence of carbon surface contamination on the GaN, which was removed by in situ exposure to an RF nitrogen plasma. RHEED observations indicate that the zincblende ZnS layers commonly contain (111) twins, although twin free films may be grown at a high substrate temperature. The samples were characterized using photoluminescence and X-ray diffraction. X-ray peaks typically had FWHM of 400 arcsec for ω/2θ scans, and somewhat worse for ω scans. Photoluminescence spectra of the ZnS films doped with Ag and Al demonstrated the well known blue donor acceptor transition at 440 nm.
STM Observation of Nitrided-Ga on Si
Authors: Nakada Y. , Miwa S. , Okumura H.
Published: April 2011

To investigate of the initial stage of GaN growth on Si, 0.2 Ga monolayers (ML) on Si (111) was nitrided and then the nitrided surfaces were observed by scanning tunneling microscopy (STM). An aggregation of islands whose longest edges had a direction rotated 15 ° from Si [110] direction was observed. The shape of islands looked like a pentagon. Surface roughness was estimated for several nitrided conditions. It was found that surface roughness becomes larger as the nitridation process proceeds.
Studies of Group III-Nitride Growth on Silicon
Authors: Blant AV , Cheng TS , Foxon CT , Bussey JC , Novikov SV , Tret’yakov VV
Published: April 2011

The growth of Group III-Nitrides on Si substrates offers the possibility of combining optoelectronics with Si technology. We have been studying the growth of Group III-Nitrides on clean and oxidised Si surfaces with a view to local area epitaxy. X-ray data indicates that alloys of (AlGa)N and (InGa)N of controlled composition can be grown on Si using a plasma enhanced Molecular Beam Epitaxy method over the entire composition range from InN to AlN. Films grown on uncleaned, oxidised surfaces of Si are polycrystalline/amorphous in contrast to growth on chemically cleaned Si substrates which show the usual columnar structure common in Group III-Nitrides. XPS studies indicate that there is little tendency for spinodal decomposition, but the In peaks in (InGa)N alloys show that more than one chemical environment is present. The composition of the alloys deduced from electron probe microanalysis studies agree well with those from X-ray measurements, assuming Vegard’s law is valid for both alloy systems.
Photoconducting Properties of Ultraviolet Detectors Based on GaN and Al1-xGaxN Films Grown by ECR-MBE
Authors: Misra M. , Korakakis D. , Singh R. , Sampath A. , Moustakas T. D.
Published: April 2011

GaN and All-xGaxN films were grown by the method of ECR-MBE. Absorption constants as a function of wavelength were determined from transmission measurements. Photoconducting detectors were fabricated from these films and characterized in terms of their spectral response and photoconductive gain. Mobility-lifetime products were determined from the measurement of photoconductive gain. The resistivity and mobility-lifetime products of the films were varied from 10-109 ohm-cm and 10-3 -10-8 cm2/V respectively by changing the microwave power in the ECR discharge from 20-60 watts. The change in the mobility-lifetime product is attributed to change in the lifetimes of the photogenerated carriers. This assumption is supported by direct measurement of detector response times. Finally, we report for the first time, the detection of alpha particles using GaN detectors.
Dislocation Luminescence in Wurtzite GaN
Authors: Shreter Y. G. , Rebane Y. T. , Davis T. J. , Barnard J. , Darbyshire M. , Steeds J. W. , Perry W. G. , Bremser M. D. , Davis R. F.
Published: April 2011

The 364 nm PL-system in GaN is attributed to the formation of dislocation excitons and charged dislocation excitons on c-axis screw dislocations. The binding energy for the dislocation exciton, charged dislocation exciton and a hole on the screw dislocation were determined as 35 meV, 7 meV and 65 meV respectively, in accord with experiment.
Photoluminescence Excitation Studies of the Optical Transitions in GaN
Authors: Kovalev D. , Averboukh B. , Meyer B. K. , Volm D. , Amano H. , Akasaki I.
Published: April 2011

We present a detailed photoluminescence excitation study of the optical transitions in GaN. This technique is employed to distinguish between band-to-band excitation and exciton contribution to the formation of the free exciton, bound exciton, violet and yellow photoluminescence bands. We show the dominant role of the Fröhlich polar intraband scattering in the formation of the free exciton states. We demonstrate that bound exciton states in a large extent are created by the capture of the free excitons by shallow impurities as well as by phononassisted resonant excitation of the bound exciton states. The capture of the free carriers excited in the band continuum is a main excitation source for the violet and yellow bands. However, distinct A- and C-exciton resonances are detected in the excitation spectra of the violet and yellow emission bands.
Structure, Electronic Properties, and Defects of GaN Using a Self-Consistent Molecular-Dynamics Method
Authors: Stumm Petra , Drabold D. A.
Published: April 2011

Molecular dynamics simulations are employed to study defects in GaN. We use local basis density functional theory within the local density approximation where charge transfer between the ions is included in an approximate fashion. We find good agreement for the band structure of wurtzite and zincblende GaN compared to other recent calculations, suggesting the suitability of our method to describe GaN. A 96 atom GaN supercell is used to study the relaxations and electronic properties of common defects in the crystal structure, including Ga and N vacancies and antisites. We analyze the electronic signatures of these defects.
Temperature Dependent Line-Shape of the Silicon Dangling Bond EPR-Resonance in Polycrystalline Silicon
Authors: Nickel N. H. , Schiff E. A.
Published: April 2011

The temperature dependence of the silicon dangling-bond resonance in polycrystalline (poly-Si) and amorphous silicon (a-Si:H) was measured. At room temperature, electron paramagnetic resonance (EPR) measurements reveal an isotropie g-value of 2.0055 and a line width of 6.5 and 6.1 G for Si dangling-bonds in a-Si:H and poly-Si, respectively. In both materials spin density and g-value are independent of temperature. While in a-Si:H the width of the resonance did not change with temperature, poly-Si exhibits a remarkable T dependence of ΔHpp. In unpassivated poly-Si a pronounced decrease of ΔHpp is observed for temperatures above 300 K. At 384 K ΔHpp reaches a minimum of 5.1 G, then increases to 6.1 G at 460 K, and eventually decreases to 4.6 G at 530 K. In hydrogenated poly-Si ΔHpp decreases monotonically above 425 K. The decrease of ΔHpp is attributed to electron hopping causing motional narrowing. An average hopping distance of 15 and 17.5 Å was estimated for unhydrogenated and H passivated poly-Si, respectively.
Annealing Studies of Visible Light Emission from Silicon Nanocrystals Produced by Implantation
Authors: Ghislotti G. , Nielsen B. , Di Mauro L. F. , Sheey B. , Mutti P. , Pifferi A. , Taroni P. , Valentini L. , Corni F. , Tonini R.
Published: April 2011

The annealing behavior of silicon implanted SiO2 layers is studied using continuous and time-gated photoluminescence (PL). Two PL emission bands are observed. A band centered at 560 nm is present in as implanted samples and it is still observed after 1000 °C annealing. The emission time is fast (0.2 -2 ns). A second band centered at 780 nm further increases when hydrogen annealing was performed. The emission time is long (1 μs - 0.3 ms).
Effect of Quantum-Well Structures on the Thermoelectric Figure of Merit in the Si/Si1-xGex System
Authors: Sun X. , Dresselhaus M. S. , Wang K. L. , Tanner M. O.
Published: April 2011

The Si/Si1-xGex quantum well system is attractive for high temperature thermoelectric applications and for demonstration of proof-of-principle for enhanced thermoelectric figure of merit Z, since the interfaces and carrier densities can be well controlled in this system. We report theoretical calculations for Z in this system, based on which Si/Si1-xGex quantum-well structures were grown by molecular-beam epitaxy. Thermoelectric and other transport measurements were made, indicating that an increase in Z over bulk values is possible through quantum confinement effects in the Si/Si1-xGex quantum-well structures.
Spectral Diffusion of Ultra-Narrow Fluorescence Spectra in Single Quantum Dots
Authors: Empedocles S. A. , Norris D. J. , Bawendi M. G.
Published: April 2011

We collect and spectrally resolve photoluminescence from single CdSe nanocrystallite quantum dots. By eliminating spectral inhomogeneities, we reveal resolution limited linewidths < 120μev at 10K. These lines are more than fifty times narrower than what has previously been reported using ensemble measurements. Light driven spectral diffusion is seen as a form of power broadening and may be the cause of surprisingly broad linewidths at room temperature. In addition, we see no evidence of excited state emission or coupling to acoustic phonons.
Cathodoluminescence of CdSe/ZnS Quantum Dot Composites
Authors: Rodriguez-Viejo J. , Heine J. R. , Dabbousi B. O. , Mattoussi H. , Michel J. , Bawendi M. G. , Jensen K. F.
Published: April 2011

We report cathodoluminescence and photoluminescence spectra originating from ZnS overcoated CdSe nanocrystals of 33 and 42 Å diameter imbedded in a ZnS matrix. The thin film quantum dot composites were synthesized by electrospray organometallic chemical vapor deposition. The cathodoluminescence intensity depends on the crystallinity of the ZnS matrix and on the voltage and current density applied. Electron beam irradiation caused a decrease of the luminescence which may be explained by electron ionization of the quantum dots. Quenching of the cathodoluminescence at low temperatures is attributed to a more efficient trapping of the ionized electrons in deep traps inside the ZnS matrix.
Optical Properties of Cuprous Oxide Nanocrystals
Authors: Rodney P. J. , Freedhoff M. I. , Marchetti A. P. , McLendon G. L. , Fauchet P. M.
Published: April 2011

Optical properties of size restricted Cu2O obtained through aqueous and non aqueous preparations are compared with those of commercially available bulk crystals. One method of synthesis involved using polyvinyl alcohol as a restraining agent in an aqueous preparation to produce nanocrystals having diameters with a mean of 11 nm and a standard deviation of 6 nm. Low-temperature spectroscopie studies indicate size restriction effects are manifest by a decrease in exciton luminescence and a 3.0 to 9.0 meV blue shift in 0,0 transition. No noticeable changes in the dynamics were observed for the nanocrystals, indicating no alteration or relaxation of the selection rules for the direct forbidden transition.
Luminescence and Surface-State Characteristics in P-Type Porous Silicon
Authors: Ramirez Porras A. , Resto O. , Weisz S. Z. , Goldstein Y. , Many A. , Savir E.
Published: April 2011

Pulse measurements on the porous-Si/electrolyte system are employed to determine the surface effective area and the surface-state density at various stages of the anodization process used to produce the porous material. Such measurements were combined with studies of the photoluminescence spectra. These spectra were found to shift progressively to the blue as a function of anodization time. The luminescence intensity increases initially with anodization time, reaches a maximum and then decreases with further anodization. The surface state density, on the other hand, increases with anodization time from an initial value of ∼2×1012 cm−2 for the virgin surface to ∼1013 cm−2 for the anodized surface. This value is attained already after ∼2 min anodization and upon further anodization remains fairly constant. In parallel, the effective surface area increases by a factor of 10–30. This behavior is markedly different from the one observed previously for n-type porous Si.
The recombination statistics of the visible photoluminescence of silicon nanocrystals
Authors: Diener J. , Kovalev D. I. , Ganichev S. D. , Polisski G. , Koch F.
Published: April 2011

A pulsed, high-power TEA CO2 laser with Unes in the region from 9.2 to 10.6 μm has been used to irradiate luminescent porous Si samples. The IR laser pulses heat the sample on a time scale much shorter than the PL decay time which is at 300 K for the PL at 1.65 eV in the order of tenth of μs. One IR pulse serves to increase the temperature of the luminescing particles in ∼2 μs up to 100°C. This increase of temperature leads to a efficient reduction of the photoluminescence (PL) intensity. However, the PL decay times are almost not affected by the heating pulse. Based on this measurement a picture of the recombination statistics that takes account of the granular Nature of the material is developed.
Preparation of Microcrystalline Silicon with the Layer-by-Layer Technique at Various Plasma Excitation Frequencies
Authors: Hapke P. , Carius R. , Finger F. , Lambertz A. , Vetterl O. , Wagner H.
Published: April 2011

For application as nucleation layer in thin film devices, microcrystalline silicon was deposited with the layer-by-layer technique using plasma excitation frequencies between 27 and 95 MHz, various hydrogen treatment times and various film thicknesses per layer. An optimum phase transformation is found at an intermediate plasma excitation frequency, i.e. at this frequency the shortest hydrogen annealing time is necessary for an effective amorphous-to-crystalline phase transformation.