bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermo-mechanical behavior of Fe thin films
Authors: Chamsoutdinov Nail R. , Böttger Amarante J. , Delhez Rob , Tichelaar Frans D.
Published: April 2011

The thermo-mechanical behavior of magnetron sputtered Fe polycrystalline films of thickness between 50 nm and 400 nm has been investigated. The state of stress has been determined by means of wafer curvature and X-ray diffraction (sin2ψ-method). Both methods are in good agreement for layers of thickness above 200 nm. For specimens of smaller layer thickness, however, the average stresses as measured by X-ray diffraction are systematically higher than those observed by wafer curvature experiments. The results can be interpreted in terms of differences in micro-strain (estimated using X-ray diffraction peak width analysis) and grain size as obtained by transmission and scanning electron microscopy. Thermal cycling experiments were performed between RT and 873 K. The effect of microstructure on thermo-mechanical properties was shown to be crucial.
Characterizing Thin-Film Stress Fields by Resonance of Membrane Arrays
Authors: Engelstad R. , Lovell E. , Chalekian A. , Janowski S. , Cash M. , Eguchi H.
Published: April 2011

Controlling thin-film stress magnitudes and nonuniformities is a persistent and pervasive challenge for applications ranging from the semiconductor industry to nanotechnology. Consequently, the ability to accurately measure the intensity and spatial distribution of film stress is essential for fabrication process optimization. This paper describes novel extensions of the membrane resonance method to characterize in-plane film stress gradients. Arrays of freestanding membrane windows are fabricated by first depositing thin films over a silicon substrate. Back-etching then creates an array of membrane windows which are sequentially resonated with a compatible drive electrode. The technique was used to determine stress uniformity across electron projection lithography masks, which incorporate membrane window arrays. In addition, a procedure is presented for identifying the two principal stresses in individual membranes.
Residual Stress in Silicon Nitride Thin Films Deposited by ECR-PECVD
Authors: Cianci E. , Foglietti V.
Published: April 2011

We have investigated the influence of process parameters in electron cyclotron resonance plasma enhanced chemical vapour deposition (ECR-PECVD) of silicon nitride, on the intrinsic stress of thin SiN films and on their composition, in order to obtain SiNx films suitable to micromechanical applications. The silane to nitrogen gas flow ratio, along with addition of helium to gas mixture, was found to be a critical parameter for the tuning of the intrinsic stress in ECR-PECVD SiNx films, from compressive to tensile stress, with a maximum related to the largest Si-N bond density in the film.
Microstructure and Current Transport Properties of YBa2Cu3O7-x/(Ba0.05, Sr0.95)TiO3 Multiple-Layer Thin Films
Authors: Luo Y. , Hughes R. A. , Preston J. S. , Botton G. A.
Published: April 2011

YBa2Cu3O7-x (YBCO) films grown by pulsed laser deposition (PLD) on (100) LaAlO3 (LAO) substrates show a strong thickness dependence on the electrical properties. For example, for films in excess of 0.3 μm, the critical current density decreases with increasing thickness. In contrast, nano-composite films consisting of a series of multiple layers of YBa2Cu3O7-x and (Ba0.05, Sr0.95)TiO3 (BSTO) thin films having a total thickness of 5 μm show improved electrical properties. In order to understand this phenomenon, a detailed microstructural characterization has been undertaken. Transmission electron microscopy (TEM) observations show that cracks, stacking faults, c-║ crystals and secondary phase precipitates are present on the single-layer films, while a high-quality microstructure is observed for the nanocomposite multiple-layer films although defects at YBCO/BSTO interface are still present. In addition, nanocomposite films have a reduced surface roughness. In this complex microstructure, the YBCO/BSTO interfaces and the lattice mismatch strain play a crucial role in controlling the nature of the defects and stability of phases. In order to understand the role of the BSTO layer has on the microstructure, the interfacial mismatch strain and defects are analyzed by high-resolution transmission electron microscopy (HRTEM) in combination with the Moiré fringe technique.
DLC/Hydroxyapatite Nanocomposites
Authors: Narayan Roger J.
Published: April 2011

Studies of orthopaedic implant failures have shown that mechanical failure of an implant almost exclusively occurs at the biomaterial-tissue interface. Hydroxyapatite (HA) mimics the behavior of natural bone, and provides a strong, long-lasting adhesive interface between a bone replacement implant and the surrounding tissue. Currently, thin film HA is not commonly used because it contains defects, porosity, and cracks. Delamination of the hydroxyapatite film and formation of hydroxyapatite particles may lead to implant wear and loosening. Furthermore, the coating also acts only as a temporary barrier to ion release from the bulk biomaterial. One method to improve the tribological properties of a bioactive coating is to strengthen the microstructure of the coating through the placement of a DLC (hydrogen-free diamondlike carbon) interlayer. DLC coatings possess properties close to diamond in terms of hardness, atomic smoothness, and chemical inertness. We have developed a diamondlike carbon/HA bilayer, in which the bilayer surface (HA) is bioactive and the interlayer (diamondlike carbon) is biocompatible, wear resistant, and corrosion resistant. We have successfully deposited nanocrystalline hydroxyapatite and DLC films by ablating a hydroxyapatite target and a graphite target using a KrF laser. A novel target design was adopted to incorporate alloying atoms into the films during pulsed laser deposition. These alloying elements possess unique biological properties. Surface morphology was studied using SEM, interfacial structure was studied using TEM, and HA phase microstructure was studied using XRD. The DLC/nanocrystalline HA bilayer material is potentially useful for several orthopedic implant designs.
Phase Field Modeling of Surface Instabilities Induced by Stresses
Authors: Seol D. J. , Hu S. Y. , Liu Z. K. , Kim S. G. , Kim W. T. , Oh K. H. , Chen L. Q.
Published: April 2011

In this work, we developed a phase field model describing surface evolution dynamics of a strained solid in contact with gas phase. Elastic solutions are solved with an iteration method for the system in which a solid film, strained by the mismatch strain between the film and substrate, has anisotropic elastic constants and the vapor phase has zero elastic constants. Elastic solutions are incorporated into the phase field evolution equation. The model predicts stable morphology at a given mismatch strain and perturbation wavelength.
Single-crystal and Nano-columnar Growth of Gadolinium-doped Ceria Thin Films on Oxide Substrates Studied Using Electron Microscopy
Authors: Huang D. X. , Chen C. L. , Jacobson A. J.
Published: April 2011

Gadolinium-doped ceria (GDC) thin films were grown by pulsed laser ablation on various oxide single crystal substrates including MgO, YSZ, LAO, NGO, and STO with different film-substrate lattice mismatch ratios. The film microstructures were characterized mainly by using electron microscopy. A clear influence of the filmsubstrate lattice mismatch on the film crystallinity has been observed. The GDC films usually exhibit columnar grain growth for a large range of film-substrate lattice mismatch ratios. A cube-on-cube growth of GDC film on MgO has been observed with a surprisingly high lattice mismatch ratio of 28%. The highest film crystallinity is obtained on the LAO substrates under a small compressive strain. This single-crystalline GDC film shows no columnar grain growth but presents a novel directionally-aligned precipitated Gd-rich nanoparticle system, which plays a specific role in relaxing various kinds of strain fields induced during the thin film growth to ensure the film crystallinity.
Young's Modulus Variation with Thickness of Thin Films
Authors: Zhou L. G. , Huang Hanchen
Published: April 2011

This paper describes atomistic determinations of the Young's modulus of free standing thin films, or nanoplates. Using a combination of analytical formulation and molecular statics simulations, we show that the Young's modulus of a nanoplate may either increase or decrease with the thickness. It is the competition of bond saturation and bond loss on surfaces that dictates the increase or decrease. Taking Cu as an example, we demonstrate that the Young's modulus is larger than its bulk counterparts for nanoplates having some surfaces and loading directions, and smaller for others.
Internal Oxidation and Mechanical Properties of Pt-IrO2 Thin Films
Authors: Chromik Richard R. , Bannuru Thirumalesh , Vinci Richard P.
Published: April 2011

Pt-IrO2 films, approximately 200 nm thick, were fabricated by co-sputter deposition of Pt and Ir in an Ar-O2 mixture followed by annealing at 700°C in O2 for 4 hours. X-ray photoelectron spectroscopy and x-ray diffraction measurements indicate the presence of IrO2 throughout the thickness of the films. After a thermal cycle in vacuum to 700°C, the room temperature residual stress is significantly lower in the internally oxidized films than in pure Pt films of similar thickness subjected to identical cycling. Initial analysis of the behavior of the films during thermal cycling indicates that the primary cause for the difference in residual stress level is a decrease in the thermoelastic slope associated with the introduction of IrO2.
Mechanisms controlling the hardness of Si and Ge
Authors: Vandeperre L. J. , Giuliani F. , Lloyd S. J. , Clegg W. J.
Published: April 2011

It is generally accepted that the hardness of silicon and germanium is limited by a phase transformation. Observations of the deformation under indents using transmission electron microscopy indicate that, in addition to the phase transformation, there is also plastic flow both in the transformed and in the untransformed material. These observations are consistent with those made elsewhere and with predictions based on the spherical cavity model for indentation, modified to quantify the effect of a phase transformation on the measured hardness. The analysis predicts that the hardness can only be approximately equal to the transformation pressure provided the yield strength of the transformed material is low, and that there is a region where plastic deformation in the untransformed material can occur in addition to the phase transformation. These predictions are consistent with the experimental observations of substantial plastic deformation of the transformed phase, as well as with estimates of its Peierls stress.
Optimization of Film Stresses Utilized in Composite Piezoelectric Membrane Microgenerators
Authors: Kennedy M. S. , Zosel M. , Richards C. D. , Richards R. F. , Bahr D. F. , Hipps K. W. , Moody N. R.
Published: April 2011

To improve the mechanical compliance of a piezoelectric generator membrane, alterations in PZT processing including variations in crystallization temperature, cooling rates and precursor chemistry were made to reduce the residual stress. In addition to changing the stresses, these treatments also altered the morphology of the PZT and its modulus. Nanoindentation of these films were made to determine the moduli of the PZT films, which varied from 70GPa to 90GPa. The residual stress of 40:60 2-MOE PZT taken by x-ray diffraction were shown to reduce in residual stress from 210 MPa to 154 MPa with annealing. The static pressure-deflection of membranes with these films showed correlating composite effective stresses of 110MPa and 83MPa. Adding PEG to the precursor solution also lowered the stress of the PZT films. Bulge testing showed that decreasing the crystallization temperature from 700°C to 600°C lowered residual stress from 210 MPa to 154 MPa. Raman spectroscopy showed differences in both stress and structure of PZT deposited on thin support structures (2μm) over bulk wafers (350 μm). The compliance of the generators was also increased by etching Pt in highly stressed regions.
Bonding Characterization of Oxidized PDMS Thin Films
Authors: McMahon J. J. , Kwon Y. , Lu J.-Q. , Cale T. S. , Gutmann R. J.
Published: April 2011

This paper reports on the use of poly(dimethylsiloxane) (PDMS) thin films to bond pairs of glass slides, borosilicate glass, and silicon substrates, with an emphasis on application for wafer-level three-dimensional (3D) heterogeneous integration technology platforms. PDMS films were spin-cast and cured, surface modified using low power oxygen plasma, and then bonded to various materials. These bonds were destructively tested using a four point bending technique. The critical adhesion energy obtained using surface modified PDMS to bond glass slides is 3.0 J/m2. Adhesion energies obtained using unmodified PDMS are 2.8, 1.8, and 1.6 J/m2 for bonded silicon, glass slides, and borosilicate glass, respectively. Correlation of these results to material surface and interface properties indicates that although PDMS has process advantages as a bonding material for heterogeneous integration, its adhesion strength is lower than that of other dielectric bonding glues.
Transparent Transistor Development
Authors: Hong D. , Dehuff N. L. , Presley R. E. , Munsee C. L. , Bender J. P. , Park C.-H. , Wager J. F. , Keszler D. A.
Published: April 2011

Transparent electronics is an embryonic technology whose objective is the realization of invisible electronic circuits. We have recently reported the fabrication of a novel n-channel transparent thin-film transistor (TTFT). [1] This ZnO-based TTFT is highly transparent and exhibits electrical characteristics that appear to be suitable for implementation as a transparent select-transistor in each pixel of an active-matrix liquid-crystal display. Moreover, the processing technology used to fabricate this device is relatively simple and appears to be compatible with inexpensive glass substrate technology. The objective of the work reported herein is to summarize some of our recent TTFT electrical performance results. Materials, processing, and device structure details related to these devices appear in future publications.
Low Resistivity Boron Doped Amorphous Silicon-Germanium Alloy Films Obtained with a Low Frequency
Authors: Heredia-J Plasma A. , Torres-J A. , De la Hidalga-W F.J. , Jaramillo-N A. , Sanchez-M J. , Zúñiga-I C. , Basurto P M. , Pérez A.
Published: April 2011

The structural and electrical properties of boron doped amorphous silicon-germanium alloy films, obtained using a low frequency plasma enhanced chemical vapor deposition (LF PECVD), are presented in this contribution. These thin films were deposited on a substrate heated at 270°C, and by decomposing a mixture of silane, germane, and diborane gases. The chemical bond structure was studied by Infrared Spectroscopy. Our results show that, for a constant diborane flow, the increase of germane flow enhances the incorporation of boron into the film; the peak at 2540 cm−1 becomes larger as the Ge content increases. Transport of carriers was studied by measuring current-voltage curves as a function of temperature. The conductivity increased from 10−6 to 10 (Ω-cm)−1, while the refraction index increased from 3.312 to 4.4458, for an increasing Ge content; this makes the films suitable for optical waveguide applications. On the other hand, the activation energy varied from 0.668 to 0.220 eV when the sample was doped with boron. The AFM images showed that the surface roughness was improved for an alloy with 50% of Ge.
Effects of Film Interfaces on the Properties of Poly-Si Grown by The Metal-Induced Technique for Solar Cell Applications
Authors: Ji Chunhai , Anderson Wayne A.
Published: April 2011

In the metal-induced growth (MIG) process, the poly-Si layer hetero-epitaxially grows from a thin silicide layer, formed by reaction of a metal seed-layer and sputtered silicon, due to an extremely close lattice match between silicon and the metal silicide. The produced poly-Si has shown a promising device quality for photovoltaic applications. Recent results show that the interface of the silicide and poly-Si has a significant effect on the properties of the poly-Si which works as an active layer for photon absorption. In the study of the MIG process, two metals were used as a seed-layer, i.e. Ni and Co. Although CoSi2has a larger lattice mismatch with Si (1.2%) than does NiSi2(0.4%), the poly-Si growing from Co has a smoother interface between the poly-Si and silicide, while the one for the Ni seed-layer samples is rather rough. Backscattered XSEM shows that the Ni-contained phase extended into the Si layer by forming long spikes. This might cause crystal defects in the Si layer. The Auger depth profile also showed that the Ni atoms diffuse into the Si layer much more than does the Co. This kind of difference in interface structure causes the different properties of the poly-Si layer. X-ray diffraction (XRD) analysis on the Si layer showed that the Co seed-layer sample had a predominant growth orientation of (220) and the FWHM of 0.2°. The Ni seed-layer samples grew mainly in both <111> and <220> direction, with FWHM of 0.3° and 0.4°, respectively. By comparison, the poly-Si from the Co seed-layer had a higher carrier lifetime of 0.458μs compared to 0.305μs from Ni.
High Power 330 nm AlInGaN UV LEDs in the High Injection Regime
Authors: Gherasimova M. , Su J. , Cui G. , Han J. , Peng H. , Makarona E. , He Y. , Song Y.-K. , Nurmikko A. V.
Published: April 2011

We report on the growth and testing of the light emitting diode structures incorporating quaternary AlInGaN active region with an emission wavelength of 330 nm. Small area circular devices were fabricated, yielding the output power of 110 μW measured with a bare-chip configuration in a high current injection regime (8 kA/cm2 for a 20 μm diameter device). Structural properties of the constituent epitaxial layers were evaluated by atomic force microscopy and transmission electron microscopy, resulting in the observation of two-dimensional growth morphologies of AlN and AlGaN, and the estimate of threading dislocation densities in the low 109 cm-2 range in the structures grown on sapphire substrates.
Optical Properties of AlN/AlGa(In)N Short Period Superlattices – Deep UV Light Emitting Diodes
Authors: Holtz M. , Ahmad I. , Kuryatkov V. V. , Borisov B. A. , Kipshidze G. D. , Chandolu A. , Nikishin S. A. , Temkin H.
Published: April 2011

We report optical properties of deep UV light emitting diodes (LEDs). Devices are based on short period superlattices of AlN/AlxGa1-x(In)N (x ∼ 0.08) grown by gas source molecular beam epitaxy with ammonia. Structures consist of a 50-nm thick AlN nucleation/buffer layer deposited on sapphire. This is followed by a 1-micron thick Si-doped buffer layer of AlGaN or AlN/AlGa(In)N designed to be transparent for wavelengths longer than 240 nm. The design thickness of the superlattice well layers is systematically varied from 0.50 nm to 1.25 nm and the thickness of the barrier is varied from 0.75 nm to 2.00 nm. The n- and p-type SPSLs were doped with Si derived from silane and Mg evaporated from an effusion cell, respectively. We investigate device structures as well as superlattices which are nominally undoped, p-type, and n-type. Optical properties are investigated using reflectance, cathodoluminescence, and, in the case of LEDs, using electroluminescence. By controlling the properties of the superlattice, we obtain optical gaps ranging from 4.5 eV (276 nm) and 5.3 eV (234 nm). A systematic shift between the optical gap and the CL peak emission energy is discussed. Electrical properties are studied using I-V, C-V, and Hall effect. LEDs based on these superlattices and operating in the range of 260 to 280 nm exhibit turn-on voltages in the range of 4 to 6 V and support dc current densities in excess of 500 A/cm2 at room temperature. We present results on the electrical and optical properties of our LEDs designed using these studies.
Proposal to Use GaAs(114) Substrates for Improvement of the Optical Transition Probability in Nitride Semiconductor Quantum Wells
Authors: Funato Mitsuru , Kawaguchi Yoshinobu , Fujita Shigeo
Published: April 2011

The dependence of the spontaneous emission lifetime of excitons in InGaN/GaN quantum disks (QDs) on the crystalline orientation is calculated. For 1-nm-thick QDs, it is found that the lifetime in the conventional c-oriented QDs is ten times as long as that in QDs tilted by 30° and 90°, and that the difference is pronounced by increasing the QDs thickness. This is totally due to the presence of the electric field in strained InGaN. Taking into account our preceding study, in which it was revealed that GaN on GaAs(114) was titled by 30°, we propose the use of GaAs(114) as a substrate for nitride light emitting devices to improve the optical transition probability.
Wafer-fused n-AlGaAs/p-GaAs/n-GaN Heterojunction Bipolar Transistor with uid-GaAs Base-Collector Setback
Authors: Estrada Sarah , Champlain James , Wang Chad , Stonas Andreas , Coldren Larry , DenBaars Steven , Mishra Umesh , Hu Evelyn
Published: April 2011

Recently we reported the first AlGaAs-GaAs-GaN heterojunction bipolar transistor (HBT), a device that potentially combines the high-breakdown voltage of an n-GaN collector with the high mobility of an AlGaAs-GaAs emitter-base. Because of the high degree of lattice mismatch between GaAs (lattice constant of 5.65A) and GaN (3.19A), we formed these devices through wafer fusion, also called direct wafer bonding. Measurements on the first generations of wafer fused HBTs revealed good current modulation, with modest output current (0.83 KA/cm2) and a current gain of 1.2. Limitations to the current gain may be related to traps and defects introduced by the fusion process, or may be a consequence of the natural conduction band offset between GaAs and GaN, which is not well known. This paper describes our new HBT structure that included a thin (20nm) uid-GaAs base-collector “setback” layer. The setback layer shifted the fused GaAs-GaN interface slightly into the collector. This new HBT structure also incorporated a reduced base thickness of 100 nm. HBTs with setback layers demonstrate increased output current (1.7 KA/cm2) and increased current gain (1.9).
Growth and Characterization of Epitaxial GaN Thin Films on 4H-SiC (11.0) Substrates
Authors: Wagner Brian P. , Preble E. A. , Reitmeier Z. J. , Davis R. F. , Zakharov D. N. , Liliental-Weber Z.
Published: April 2011

GaN thin films were grown via metalorganic vapor phase epitaxy on a-plane 4H-SiC substrates on which had been deposited an AlN buffer layer. Atomic force microscopy images revealed that the microstructure of the AlN buffer layer and the subsequently deposited GaN had a highly oriented growth structure where parallel growth features propagated in the [1–1.0] direction. Scanning electron microscopy showed that the interfaces between the substrate, buffer layer, and epi-layer were continuous. Cracking was observed in GaN films having a thickness greater than 800 nm. Plan-view transmission electron microscopy analysis revealed stacking faults and threading dislocations with densities of ∼1.6×106cm-1 and ∼ 3.3×1010cm-1, respectively. X-ray diffraction confirmed that the GaN was deposited epitaxially in the same orientation as the substrate. The average on- and off-axis x-ray full-width half-maxima of the (11.0) and the (10.0) reflections were 948 arcsec and 5448 arcsec, respectively.
GaN layers re-grown on etched GaN templates by plasma assisted molecular beam epitaxy
Authors: He L. , Xie J. , Yun F. , Baski A. A. , Morkoç H.
Published: April 2011

The growth of high-quality GaN by plasma assisted molecular beam epitaxy (MBE) is challenging, in part due to the constraint of heteroepitaxy since GaN substrates are not yet commercially available and isotropic nature of growth. Despite the large lattice and thermal mismatch between sapphire and GaN, the former is still the most commonly used substrate for the GaN-based optical devices at present. In this paper, we demonstrate a re-growth technique to obtain an improved quality GaN by MBE on GaN template on sapphire where the grossly defective regions have been removed. This GaN template is formed by MBE growth of GaN followed by wet chemical etching to selectively remove the defective region. Improved quality GaN was re-grown on such a template under Ga rich conditions to a thickness of about 1 micron. After re-growth, the surface of GaN is atomically smooth with spiral features in the short range. The low temperature PL of the re-grown GaN is superior to those of MBE GaN films directly on sapphire. Atomic force microscopy (AFM) images reveal a two-dimensional re-growth initiating in regions free of extended defects. The results show that the selectively etched GaN on sapphire can be used as a good template to improve the quality of GaN.
Properties of Crucible Materials for Bulk Growth of AlN
Authors: Slack Glen A. , Whitlock Jon , Morgan Ken , Schowalter Leo J.
Published: April 2011

A variety of different crucible materials have been suggested and/or employed for the sublimation-recondensation growth of AlN single crystals above 2000 C. Representative materials all have melting points well above 2300 C, a reasonable degree of chemical compatibility with AlN, relatively low vapor pressures, and relatively small thermal expansion coefficients. We analyze the current state of knowledge on crucible materials such as C, W, Re, W-Re alloys, BN, HfN, HfC, NbC, TaC, Ta2C, TaN, ZrC and ZrN with respect to published bulk AlN growth conditions. Crucible materials pyrolytic graphite, pyrolytic BN, and W have integrated thermal contraction values (upon cooling from growth temperatures) that are less than that of AlN; the other materials have larger values. The lowest vapor pressure materials in a nitrogen atmosphere are W, TaC, and Re; thus they are expected to yield higher purity crystals than the other candidates. The materials C, BN, Hf, and ZrN are expected to contribute to higher impurity levels in the AlN crystals.
Influence of metal thickness to sensitivity of Pt/GaN Schottky diodes for gas sensing applications
Authors: Tilak V. , Ali M. , Cimalla V. , Manivannan V. , Sandvik P. , Fedison J. , Ambacher O. , Merfeld D.
Published: April 2011

Hydrogen gas sensors based on Pt/GaN Schottky diode structures were fabricated and their responses to hydrogen were studied. These diodes were fabricated on Si doped GaN layer (ND = 1×1017). Three sets of diodes were fabricated with 80 Å, 240 Å and 400 Å of Pt for the Schottky contacts. The electronic performances of 0.25 × 0.25 mm devices were tested in up to 1 % H2 gas in synthetic air (79% N2, 21% O2) by volume. The devices were operated in constant current mode in a forward bias condition. The change in voltage was monitored with the diodes exposed to hydrogen and to dry air at varying temperatures. The responses increased as the thickness of the Schottky metal contact decreased at any given temperature up to 310 °C. The trend of increasing response with decreasing thickness was also observed in 0.5 × 0.5 mm and 1.0 × 1.0 mm size Schottky diodes. SEM studies of the microstructure showed that the thinner Pt devices had higher grain boundary densities. The increase in sensitivity with decreasing thickness points to the dissociation of molecular hydrogen on the surface, the diffusion of atomic hydrogen through the Pt grain boundaries and the adsorption of hydrogen to the surface as a possible mechanism of sensing of hydrogen by Schottky diodes.
Band-GaP Energy and Physical Properties of InN Grown by RF-Molecular Beam Epitaxy
Authors: Nanishi Yasushi , Saito Yoshiki , Yamaguchi Tomohiro , Matsuda Fumie , Araki Tsutomu , Naoi Hiroyuki , Suzuki Akira , Harima Hiroshi , Miyajima Takao
Published: April 2011

This paper describes studies on high-quality InN growth on sapphire by RF-MBE. Critical procedures to obtain high-quality InN films were investigated and (1) nitridation process of sapphire substrates prior to growth, (2) precise control of V/III ratio and (3) selection of optimum growth temperature were found to be essential. Detailed structural characterizations by XRD, TEM, Raman scattering and EXAFS indicate that InN films obtained in this study have ideal hexagonal wurtzite structure. FWHMs of ω-2Θ mode XRD and E2(high)-phonon-mode of Raman scattering are as small as 28.9 arcsec and 3.2 cm-1, respectively. True band gap energy of InN is also discussed based on optical characterization results obtained from well-characterized hexagonal InN grown in this study. PbS, instead of InGaAs, was used as a detector for PL study in order to solve the problem coming from the cut-off wavelength of InGaAs detector. Based on these systematic studies on structural and optical property characterizations using high-quality InN, true band-gap energy of InN is suggested to be less than 0.67 eV and approximately 0.65 eV at room temperature. Single-crystalline InN films are also successfully grown on Si substrates by a brief nitridation of the Si substrates. Significant improvement of InN crystal quality on Si substrates by the insertion of an AlN buffer layer is also demonstrated.
Growth and Surface Reconstructions of AlN(0001) Films
Authors: Lee C. D. , Dong Y. , Feenstra R. M. , Northrup J. E. , Neugebauer J.
Published: April 2011

The growth and surface reconstructions of AlN(0001) films were studied. For moderately Al-rich surfaces, the 2×6 structure is commonly observed in reflection high energy electron diffraction. It is found that this pattern consists of 2√3×2√3-R30° and 5√3×5√3-R30° reconstructions according to scanning tunneling microscopy. Similar to the Ga-rich GaN(0001) surface, these structures are determined to contain 2–3 monolayers of excess Al terminating the surface. Based on first-principles theory the structures are believed to contain a laterally contracted Al layer. At higher Al coverage a thick, flat Al film is found to form on the surface. A high density of growth spirals (associated with threading dislocations having full or partial screw character) is found to be present on the surface, although this density decreases with increasing growth temperature.
Hydrogen-related local vibrational modes in GaN:Mg grown by molecular beam epitaxy
Authors: Pastor D. , Cuscó R. , Artus L. , Naranjo F. , Calleja E.
Published: April 2011

We report a Raman scattering study of local vibrational modes (LVMs) on Mg-doped GaN grown by molecular beam epitaxy (MBE). Besides Mg:Ga local vibrational modes clearly observed at 262 and 565 cm−1, several peaks were detected in the spectral regions around 2200 cm−1 and 2900 cm−1. The modes in the 2200 cm−1 spectral region correspond to local modes of hydrogen complexes and hydrogen-decorated defects, and indicate the presence of a fairly high concentration of H in the samples. The peaks observed in the 2900 cm−1 region are assigned to carbon-hydrogen local modes and are indicative of the presence of C impurities in the samples. These measurements show that both C and H impurities may be present in sizable amounts not only in metal-organic chemical vapor deposition (MOCVD) samples but also in MBE grown samples, and this may have an effect on the electrical conductivity ofp-type GaN:Mg samples.
Surface Potential Measurements of doping and defects in p-GaN
Authors: Losurdo M. , Giangregorio M. M. , Bruno G. , Brown A. S. , Doolittle W. A. , Namkoong Gon , Ptak A. J.
Published: April 2011

The interaction of Be-, Mg-, and Si- doped GaN epitaxial films with atomic hydrogen, produced by a remote r.f. hydrogen plasma, is investigated. The kinetics of the interaction is monitored in real time by spectroscopic ellipsometry through the measurement of the variation of the GaN pseudodielectric function. The passivation effect of hydrogen is inferred by surface potential measurements using scanning Kelvin probe microscopy (SKPM). It is found that the interaction of GaN with hydrogen is a strong function of both the type and level of the doping. Hydrogen treatment is shown to lead to a strong variation of the surface potential and, hence, of the Fermi level position, which is the result of p-dopant passivation by hydrogen. A different interaction of Mg and Be with atomic hydrogen is also observed and monitored in real time by ellipsometry. SKPM is also used for studying the interaction of defects in GaN with atomic hydrogen.
Electron micro-probe analysis and cathodoluminescence spectroscopy of rare earth - implanted GaN
Authors: Dalmasso S. , Martin R. W. , Edwards P. R. , Katchkanov V. , O'Donnell K. P. , Lorenz K. , Alves E. , Wahl U. , Pipeleers B. , Matias V. , Vantomme A. , Nakanishi Y. , Wakahara A. , Yoshida A.
Published: April 2011

GaN films doped with rare earth (RE) elements have attracted considerable attention due to the unique optical luminescent properties of the RE intra 4ƒ(n)-shell electron transitions which lead to sharp blue (Tm), green (Er) and red (Eu) emissions. This paper presents an overview of investigations of GaN films implanted with each of these ions using a combination of electron-beam and optical techniques. The ion implantations were performed under a wide range of conditions, covering variations in fluences, energies and temperatures and followed by different high-temperature annealing steps. The resulting RE:GaN films were analysed using an electron probe micro-analyser modified to allow cathodoluminescence (CL) spectroscopy. Elemental microanalysis data obtained by wavelength dispersive X-ray analysis (WDX) is correlated with simultaneously collected room temperature CL spectra.

WDX allows the quantification of the RE elemental concentrations in GaN down to ∼ 0.03 at. % in very thin layers (∼ 100 nm deep). Furthermore, by varying the incident electron beam energy, details concerning the depth profile of RE implants can be determined. The effects of both implantation conditions and rapid thermal annealing on the depth profile and on the luminescence properties are reported. CL measurements performed on annealed samples reveal sharp visible and near IR emission lines characteristic of the RE3+ intra-4ƒ(n) atomic shell transitions.
Formation and dissociation of hydrogen-related defect centers in Mg-doped GaN
Authors: Gelhausen O. , Phillips M. R. , Goldys E. M. , Paskova T. , Monemar B. , Strassburg M. , Hoffmann A.
Published: April 2011

Moderately and heavily Mg-doped GaN were studied by a combination of post-growth annealing processes and electron beam irradiation techniques during cathodoluminescence (CL) to elucidate the chemical origin of the recombination centers responsible for the main optical emission lines. The shallow donor at 20–30 meV below the conduction band, which is involved in the donor-acceptor-pair (DAP) emission at 3.27 eV, was attributed to a hydrogen-related center, presumably a (VN-H) complex. Due to the small dissociation energy (<2 eV) of the (VN-H) complex, this emission line was strongly reduced by low-energy electron irradiation. CL investigations of the DAP at a similar energetic position in Si-doped (n-type) GaN indicated that this emission line is of different chemical origin than the 3.27 eV DAP in Mg-doped GaN. A slightly deeper DAP emission centered at 3.14 eV was observed following low-energy electron irradiation, indicating the appearance of an additional donor level with a binding energy of 100–200 meV, which was tentatively attributed to a VN-related center. The blue band (2.8–3.0 eV) in heavily Mg-doped GaN was found to consist of at least two different deep donor levels at 350±30 meV and 440±40 meV. The donor level at 350±30 meV was strongly affected by electron irradiation and attributed to a H-related defect.
Microstructure of Nonpolar a-Plane GaN Grown on (1120) 4H-SiC Investigated by TEM.
Authors: Zakharov D. N. , Liliental-Weber Z. , Wagner B. , Reitmeier Z. J. , Preble E. A. , Davis R. F.
Published: April 2011

Plan-view and cross-section samples of (1120) (a-plane) GaN grown on 4H-SiC substrates with AlN buffer layers were studied by transmission electron microscopy. Samples reveal the presence of a high density of stacking faults formed on the basal plane of hexagonal GaN. These stacking faults, terminated in the growth plane by threading dislocations, nucleate at the AlN/4H-SiC interface and propagate to the GaN layer surface. High resolution electron microscopy shows that the majority of stacking faults are low-energy planar defects of the type I1and I2. High energy stacking faults (E) are not observed.
Structure and electrical activity of rare-earth dopants in selected III-Vs
Authors: Filhol J.-S. , Petit S. , Jones R. , Hourahine B. , Frauenheim Th. , Overhof H. , Coutinho J. , Shaw M. J. , Briddon P. R. , Öberg S.
Published: April 2011

Density functional theory is used to investigate Eu, Er and Tm rare earth (RE) impurities in GaAs, GaN and AlN. The most stable site is when the RE is located at a group III substitutional site but in GaN and GaAs these defects do not then possess any gap levels, unlike AlN. RE-VN defects in GaN are shown to possess levels which could act as traps for excitons. The interaction of oxygen with substitutional REs is also considered.
Identification of Carbon-related Bandgap States in GaN Grown by MOCVD
Authors: Armstrong A. , Arehart A. R. , Ringel S. A. , Moran B. , DenBaars S. P. , Mishra U. K. , Speck J. S.
Published: April 2011

Carbon doping of GaN is of great interest in part because it has been shown to result in semi-insulating (SI) behavior. However, determination of the bandgap states and hence the exact mechanism responsible for the SI behavior is, to date, an unresolved issue. A key impediment is that the presumed C acceptor levels are likely near the minority carrier (valence) bandedge of otherwise background n-type GaN, and hence their precise detection by usual methods is difficult. In this paper, we exploit the inherent ability of deep level optical spectroscopy (DLOS) to detect states near the minority carrier band edge, as well as potentially deep states associated with C in background n-type GaN. This is accomplished by comparing unintentionally doped (uid) GaN grown by atmospheric pressure (AP) MOCVD, which has residual n-type conductivity, with LP MOCVD GaN that incorporates a large concentration of C for both uid and intentionally Si co-doped conditions. The results show the emergence of a shallow state at Ec - 3.28 eV (Ev + 0.16 eV) for the LP samples with a minimum concentration of 3.6 × 1016 cm-3 that efficiently compensates Si donors for the co-doped sample, and results in semi-insulating behavior for the uid-LP sample. In contrast, this state is not observed for the AP GaN material, which incorporates a factor of ∼10 times less C, and instead only the expected residual Mg acceptor level at Ec - 3.22 eV is observed. Additionally, a state at Ec - 1.35 eV, near the theoretically expected C split-interstitial level in n-type GaN, is observed to increase significantly in concentration with increased C concentration.
Raman Characterization of Strained GaNyAs1-y and InxGa1-xNyAs1-y Epilayers
Authors: Tay Li-Lin , Lockwood David J. , Gupta James A. , Wasilewski Zbig R.
Published: April 2011

Pseudomorphically strained epitaxial films of the ternary alloy GaNyAs1-y have been grown on GaAs(100) with y ranging from 0 to 0.05. The optical phonon Raman spectrum of the alloy displays a two-mode behavior. The GaAs-like first order modes are represented at y = 0.05 by the strong longitudinal optic (LO1) mode at 288.5 cm-1 and the weaker transverse optic (TO1) mode at 268.3 cm-1, while the GaN-like LO2 mode is observed at 474.8 cm-1. Two very broad disorder-induced acoustic bands are evident at 80 and 170 cm-1 due to atomic disorder within the crystalline network. Raman studies show that as the nitrogen concentration increases, the GaAs-like LO1 band shifts linearly towards lower wavenumber while the GaN-like LO2 phonon band displays a sub-linear increase in wavenumber. Raman results for the unstrained quaternary alloy In0.06Ga0.94N0.02As0.98 are compared with those of GaN0.02As0.98.
Luminescence Properties of Eu ion-implanted GaN
Authors: Uekusa Shin-ichiro , Tanaka Isao
Published: April 2011

The Eu ion was implanted at an energy of 300keV with a dose of 1×1015cm-2 at room temperature. Photoluminescence (PL) and PL lifetime were characterized and studied on thermal quenching process. We calculated the activation energy (E1) of temperature dependent PL and the value of E1 was 5.68meV. E1 was affected the luminescence intensity in the temperature range from 15K to 70K. The activation energy (Ea) of PL lifetime was calculated and the value of Ea was 8.5meV. The non-radiative recombination in the transition from the 5D0 to 7F2 of Eu was dominated in the temperature range from 15K to 100K. We found that the thermal quenching occurred in both the electron emission from RE-trap and the non-radiative recombination in the transition on Eu in the temperature range from 15K to 70K.
Origins of Light Emission and Efficiency Saturation of the Photoluminescence of GaN Nanocrystallites
Authors: Chen Xiang-Bai , Morrison John L. , Penner Margaret K. , Elle Jennifer , Bergman Leah , Purdy Andrew P.
Published: April 2011

The photoluminescence (PL) properties of GaN nanorods were studied utilizing UV micro-photoluminescence. The room temperature PL of the GaN nanorods exhibits one strong emission line. The PL intensity as a function of the laser power was investigated in order to determine whether this emission originates from an excitonic or a bandgap recombination process. Our analysis indicates that the PL of the rods is excitonic-like and very similar to the behavior of the free exciton A in GaN thin films. However, for a relatively large and compact ensemble of rods, the PL intensity exhibits a significant saturation occurring already at relatively low laser power. We attribute the intensity saturation to the laser heating and heat trapping which takes place in the enclosure of the ensemble.
Polarization-dependent spectroscopy of the near-bandgap excitonic emission in free standing GaN
Authors: Paskov P. P. , Paskova T. , Holtz P. O. , Monemar B.
Published: April 2011

We report a comparative study of the exciton emission in free standing HVPE layer for all polarization configurations. A noticeable difference between the emission spectra polarized perpendicular and parallel to the c-axis of the crystal is observed. The spectra for E┴c and EIIc are found to be dominated by the emissions of the donor-bound exciton and exciton-polariton both arising from the A and B valence band, respectively, which clearly reveals the optical selection rules in wurtzite GaN. The temperature evolution of the emission spectra is also examined and the thermal redistribution of the excitons at different polarization is discussed.
Structural and Optical Characterization of InGaN Layers Grown by MOMBE
Authors: Singh P. , Aderhold J. , Graul J. , Davydov V. Yu. , Gourbilleau F. , Ruterana P.
Published: April 2011

The investigated InGaN ternary films were grown by MOMBE, with the In fraction varying from 6% to 100%. Using XRD and TEM, we determine the crystalline state, the nature of defects inside the layers and their distribution as related to the In concentration. The band gap of the alloys was studied through PL measurements, it is below 1 eV for InN.
Growth and Characterization of GaPNAs on Si
Authors: Geisz John , Olson J. M. , McMahon W. E. , Hannappel T. , Jones K. , Moutinho H. , Al-Jassim M. M.
Published: April 2011

The dilute nitrogen alloy GaPNAs can be lattice matched to silicon with band gaps ranging from 2.3 eV to less than 1.7eV making it of special interest for photovoltaic applications. We have studied the growth and structural quality of the alloy grown on vicinal Si(001) and GaP(001) substrates by MOCVD. Using a particular nucleation scheme, we have deposited 1-μm thick layers that are crack-free and exhibit narrow x-ray line widths. The FWHM of the (004) x-ray reflection from a GaP1−xNx epilayer decreases dramatically from ∼300 arcsec for x=0 to 18 arcsec for x = 0.021. The band gap of this alloy is 1.96 eV. With the addition of As (and more N), the x-ray line widths tend to increase slightly to 27 arcsec.
Analysis of Emission Rate Measurements in a Material Showing a Meyer-Neldel- Rule
Authors: Crandall Richard S.
Published: April 2011

This paper presents data showing a Meyer-Neldel rule (MNR) in InGaAsN alloys. It is shown that without this knowledge, significant errors can be made using Deep-Level Transient-Spectroscopy (DLTS) emission data to determine capture cross sections. The errors arise because of the neglect of significant transition entropy changes associated with multiphonon excitation of charge from deep traps. Ignoring the entropy change results in cross section values ranging over five orders-of-magnitude in InGaAsN alloys and 18 orders-of-magnitude in CuInGaSe alloys. Only by correctly accounting for the MNR and the accompanying entropy changes in analyzing the DLTS data will the correct value of the cross section be obtained.
HVPE-based orientation-patterned GaAs: added-value for non-linear applications.
Authors: D. Faye , E. Lallier , A. Grisard , B. Gérard , E. Gil-Lafon
Published: April 2011

A new route is presented for the development of Orientation-Patterned Gallium Arsenide (OP-GaAs) for both guided and non-guided optical applications. The method is based on the use of the near-equilibrium growth process HVPE (Hydride Vapour Phase Epitaxy) that enables thick, high quality and orientation-preserving regrowth over dimensions fully compatible with targeted applications. The control of the growth anisotropy permits to preserve the original periodic arrangement of the initial template so that OP-GaAs substrates can be fabricated with pattern width and structure thickness ranging respectively from 1.8 to 110μm and 20 to 500μm. Such a large scope of dimensions appears useful either for bulk structures such as Optical Parametric Oscillators (OPO) or telecommunication applications. As an example, we suggest that combining this growth method with mechano-chemical polishing process could decrease propagation losses on AlGaAs waveguide structures for frequency conversion in telecom bands.
Estimates of Impact Ionization Coefficients in Superlattice-Based Mid-Wavelength Infrared Avalanche Photodiodes
Authors: Grein C. H. , Abu El-Rub K. , Flatté M. E. , Ehrenreich H.
Published: April 2011

We describe band engineering strategies to either enhance or suppress electron-initiated impact ionization relative to hole-initiated impact ionization in type II superlattice mid-wavelength infrared avalanche photodiodes. The strategy to enhance electron-initiated impact ionization involves placing a high density of states at approximately one energy gap above the bottom of the conduction band and simultaneously removing valence band states from the vicinity of one energy gap below the top of the valence band. This gives the electrons a low threshold energy and the holes a high one. The opposite strategy enhances hole-initiated impact ionization. Estimates of the electron (α) and hole (β) impact ionization coefficients predict that α/β>>1 in the first type of superlattice and α/β<<1 in the second type.
A New Class of Solar Cells: Isomeric Boron Carbide Semiconductors with Fourth Quadrant Conductivity
Authors: Billa Ravi B. , Caruso A. N. , Brand J. I.
Published: April 2011

Previously, we have made diodes[1,2] and transistors [3] as well as very effective realtime solid state neutron detectors [4] out of semiconducting boron carbide deposited on silicon or silicon carbide‥

In this work the recent fabrication of a new class of highly photosensitive boron carbide diodes is discussed. These diodes exploit the electronic behavior differences of the isomers of film precursors, the closo-dicarbododecaboranes. These differences were observed in photoemission and inverse photoemission studies where the HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) gap variations upon deposition varied strongly with the isomeric configuration. Based on these results, p-n junctions were formed by plasma enhanced chemical vapor of ortho and meta carborane, respectively, on both nickel and aluminum substrates. These diodes exhibit fourth-quadrant conductivity, making them exciting new photovoltaic conversion devices.