bluuebunny/crossref_metadata_2025 huggingface dataset:
Space Radiation Effects in Advanced Solar Cell Materials and Devices
Authors: Walters R. J. , Summers G. P.
Published: April 2011

An investigation of the physical mechanisms governing the response of III-V based solar cells to particle irradiation is presented. The effect of particle irradiation on single and multijunction solar cells is studied through current vs. voltage, spectral response, and deep level transient spectroscopy measurements. The basic radiation response mechanisms are identified, and their effects on the solar cell electrical performance are described. In particular, a detailed analysis of multijunction InxGa1-xP/InyGa1-yAs/Ge devices is presented. The MJ cell response is found to be more strongly affected by the internal cell structure than by the In content.
The Influence of Annealing Temperature and Doping on the Red/Near-Infrared Luminescence of Ion Implanted SiO2:nc-Si
Authors: Tetelbaum D. I. , Burdov V. A. , Trushin S. A. , Mikhaylov A. N. , Revin D. G. , Gaponova D. M.
Published: April 2011

The results of an experimental research of the dependence of photoluminescence (PL) intensity in region about 800 nm for silicon nanoinclusions (quantum dots) obtained by Si ion implantation in SiO2 on the dose of Si ions at two temperatures of an annealing ?ann = 1000 and 1100° ? are presented. It is established that in both cases the dependences have the shape of the curves with a maximum. For 1100° ? the maximum is shifted to the lower dose. The influence of an additional ion doping by the phosphorus on intensity of PL is investigated depending on the dose (concentration) of P and the dose of the silicon at ?ann = 1000° ?. It is shown, that in all the investigated region of P doses, the presence of P enhances the PL. The degree of the enhancement increases with the P dose, but the rate of the intensity enhancement goes down. With the growth of Si dose at the constant dose of P, the degree of the enhancement decreases. In an approximation of an effective mass, the energy spectra of a quantum dot are calculated at the presence of one or several P atoms for various their arrangement.
Electronic structure near the band gap of heavily nitrogen doped GaAs and GaP
Authors: Zhang Yong , Fluegel B. , Hanna M. , Duda A. , Mascarenhas A.
Published: April 2011

Isoelectronic impurity nitrogen atoms have been found to generate a series of localized states in GaP and GaAs. These states can be either bound (within the band gap) or resonant (above the band gap) when in the dilute doping limit (roughly < 1019 cm−3 for GaP and < 1018 cm−3 for GaAs). With increasing nitrogen doping level, a shift of the absorption edge from the binary band gap has been observed for the so-called GaPN or GaAsN alloy. We discuss the similarity and dissimilarity between the two systems in the following aspects: (1) How does the nitrogen doping perturb the host band structure? (2) How do the nitrogen bound states evolve with increasing nitrogen doping level? (3) What are the dominant contributors to the band edge absorption? And (4) does a universal model exist for GaPN and GaAsN? Other issues that will be discussed are: how does one define the band gap for these materials, and what is the relevance of various theoretical band structure calculations to the experimentally measured parameters.
Enhanced Photoluminescence from Long Wavelength InAs Quantum Dots Embedded in a Graded (In,Ga)As Quantum Well
Authors: Chen L. , Stoleru V. G. , Pal D. , Pan D. , Towe E.
Published: April 2011

Three sets of self-organized InAs quantum dots (QDs) embedded in an external InGaAs quantum well samples were grown by solid source molecular beam epitaxy (MBE). By modifying Indium composition profile within quantum well (QW) region, it's found the photoluminescence emission from quantum dots can be greatly enhanced when employing a graded quantum well to surround QDs. This quantum dots in a graded quantum well structure also preserves the long wavelength (1.3 μm) spectrum requirement for the future use in optoelectronics devices.
GaAs Photodetector for X-ray Imaging
Authors: Sun G. C. , Samic H. , Donchev V. , Gautrot S. , Bourgoin J. C.
Published: April 2011

We describe briefly a cheap and non polluting technique to grow epitaxial GaAs layers, several hundred microns thick, in a matter of hour. Detectors consisting of a p+/i/n+ structure have been realised with these layers and we present their characteristics obtained from currentvoltage, capacitance-voltage measurements as well as their response versus the energy and flux of X-rays.
Intersubband Transitions In Ingaas/Inalas Multiple Quantum Wells Grown On Inp Substrate
Authors: Zhou Qiaoying , Manasreh M. O. , Weaver B. D. , Missous M.
Published: April 2011

Intersubband transitions in In0.52Ga0.48As/In0.52Al0.48As multiple quantum wells grown on lattice matched InP substrates were investigated using Fourier transform infrared (FTIR) absorption and photoluminescence (FTPL) techniques. The well width was tailored to produce excited states resonant in the conduction band, at the edge of the conduction band, and confined in the quantum wells. Interband transitions were also probed using FTPL and optical absorption techniques. The FTPL spectra show that three interband transitions exist in the quantum well structures with well width larger than 30 Å. The intersubband transitions in this class of quantum wells seem to withstand proton irradiation with doses higher than those used to deplete the intersubband transitions in the GaAs/AlGaAs multiple quantum wells.
Structural Properties and Doping of Zn1-x(Mg,Li)xOMaterials
Authors: Melgarejo R. E. , Tomar M. S. , Hidalgo A. , Katiyar R. S.
Published: April 2011

The possibility of the solid solutions with MgO provides the opportunity in ZnO/ZnMgO quantum well structures. The recent observation that n-type conductivity in ZnO is due to hydrogen doping open the avenue for p-doping to form p-n junction devices in ZnO.. We report on the synthesis and structural properties of Zn (Mg,Li)O. The possibility of p-doping by P has been explored.
New Approach Towards The Deposition of I-III-VI Thin Films
Authors: Afzaal Mohammad , Crouch David , O'Brien Paul , Park Jin-Ho
Published: April 2011

The ternary chalcopyrite semiconductor Cu(In/Ga)(Se/S)2 is currently used as an absorber layer in high efficiency thin film solar cells. In this study, various types of I-III-VI (I = Cu, III = Ga or In, VI = S or Se) thin films (CuGaS2, CuInS2 and CuInSe2) were prepared from a series of organometallic precursors, M[(S/Se)2CNMeR]n (M = Cu, In, Ga; R = alkyl) by aerosol-assisted chemical vapour deposition (AACVD). In contrast to the metal alkyl compounds, MR3 (M = In and Ga; R = alkyl), which are pyrophoric, the precursors are easy to synthesize by one-pot reactions and are air stable. The optimum growth temperature for the preparation of these films on glass substrates using aerosol-assisted chemical vapour deposition (AACVD) was found to be above 400 °C in terms of crystallinity, although deposition does occur at lower temperatures. The films have been characterised using XRPD, SEM and EDS. SEM analyses show all films are microcrystalline. XRPD results show evidence of the crystalline nature of theses films. The results of this comprehensive study are presented and discussed.
Local Vibrational Modes of Carbon-Hydrogen Complexes in Proton Irradiated AlGaN
Authors: Manasreh M. O. , Weaver B. D.
Published: April 2011

Local vibrational modes (LVMs) of carbon-hydrogen (C-H) complexes in proton implanted AlGaN grown on sapphire by metalorganic chemical vapor deposition (MOCVD) were investigated using Fourier transform infrared (FTIR) spectroscopy. The LVMs exhibit five distinctive peaks in the spectral region of 2846∼2963 cm−1, which are due to the symmetric and asymmetric stretching modes of C-Hn (n=1–3). The LVMs intensities in doped AlGaN are increased as irradiation dose is increased in the entire irradiation dose range used without reaching the saturation stage. On the other hand, undoped samples show that LVMs intensity increase then either decreased or saturated as the irradiation dose is increased above 5×1016 cm−2. Proton irradiation causes a drastic increase in the CH3 LVM while electron irradiation causes the opposite effect suggesting strongly that the observed LVMs are truly due to CH complexes.
I-V And C-V Characteristics of nGaAs-nInSb Heterojunctions Obtained by Pulsed Laser Deposition Technique
Authors: Avdjian Karapet E.
Published: April 2011

Using pulsed laser deposition technique, nGaAs-nInSb heterojunctions (HJs) are obtained. Their electrical properties are studied. The Current-Voltage characteristics show that obtained HJs possess rectifying properties. The rectification coefficient depends strongly on the doping level of GaAs substrate. The linear dependence of C−2 (V) curve in Capacitance-Voltage characteristics, as well as the change of photo-response sign with wavelength, indicates that the HJs have abrupt interface, which is, to the best of our knowledge, a novel result for these HJ materials. The full number of interface states arising due to the lattice mismatch is determined which is in agreement with Hall measurements. Current-Voltage characteristics of obtained HJs are analogous to those of metal-semiconductor junction. Based on the obtained results the energy band diagrams of nGaAs-nInSb HJ is constructed taking into account the interface states.
Development of AIIBVI Semiconductors Doped with Cr for IRLaser Application
Authors: Kasiyan V. A. , Shneck R. Z. , Dashevsky Z. M. , Rotman S. R.
Published: April 2011

Electrical and optical measurements obtained with CdSe single crystals doped with chromium from a gas source CrSe over a wide temperature range (500–1050 °C) are compared with ZnSe annealed in liquid metal (Zn). These processes are intended to control the concentrations of the impurity and intrinsic defects. The low temperature annealing of CdSe crystals in CrSe atmosphere allows obtaining high electron mobility up to 9000 cm2/Vs at 80 K and demonstrates the low native defect concentration. A high temperature annealing gives rise to increased electron concentration with decreased mobility. Optical absorption measurements show that at the high annealing temperature effective doping with Cr takes place. The impurity absorption beyond the absorption edge is interpreted by the excitation of Cr++ and Cr+ deep levels
Low-Resistance Electrical Contacts to p-Type GaN by Using InxGa1-xN Cap Layers
Authors: Gessmann Th. , Li Y.-L. , Graff J. W. , Schubert E. F. , Sheu J. K.
Published: April 2011

A novel type of low-resistance ohmic contacts is demonstrated utilizing polarization-induced electric fields in thin p-type InGaN layers on p-type GaN. An increase of the hole tunneling probability through the barrier and a concomitant significant decrease of the specific contact resistance can be attributed to a reduction of the tunneling barrier width in the InGaN capping layers due to the polarization-induced electric fields. The specific contact resistance of Ni (10 nm) / Au (30 nm) contacts deposited on the InGaN capping layers was determined by the transmission line method. Specific contact resistances of 1.2 × 10-2 Ω cm2 and 6 × 10-3 & cm2 were obtained for capping layer thicknesses of 20 nm and 2 nm, respectively.
High common-emitter current gains obtained by pnp GaN bipolar junction transistors
Authors: Kumakura Kazuhide , Makimoto Toshiki , Kobayashi Naoki
Published: April 2011

We fabricated pnp GaN bipolar junction transistors and investigated their common-emitter and common-base current-voltage characteristics. The device structures were grown by metalorganic vapor phase epitaxy on a sapphire substrate. The base thickness was 0.12 μm and its electron concentration was estimated to be 3 × 1017 cm-3 from the common-emitter current-voltage characteristics and the base conductivity. The common-emitter current-voltage characteristics showed very low leak current. The maximum current gains at room temperature were 50 and 69 from the common-emitter and the common-base current-voltage characteristics, respectively.
Low Resistance Non-Transparent ohmic Pt-contacts on p-GaN
Authors: Weimar Andreas , Bader Stefan , Brüderl Georg , Kümmler Volker
Published: April 2011

The metal – p-GaN junction for low resistance ohmic contacts is still a challenge to be applied in GaN-based opto electronics as well as in power and high frequency devices. Currently, we try to improve the performance of our blue laser diodes. In order to decrease heat generation during device operation it is necessary to ensure as small contact resistances as possible.

In this work, we achieved a specific contact resistance value of RC = 1.8 ± 1.7. 10-5 Ωcm2 for Pt-contacts on MOVPE-grown p-GaN. The Pt-layers were deposited by e-beam and thermally assisted vacuum evaporation after a standard cleaning process. For evaluation of Rc we used optimised circular TLM test patterns defined by photolithography. Best contacts were formed by annealing in Nitrogen athmosphere at 500°C.

We also investigated the dependence of the contact resistance on the Mg doping concentration. Therefore p-GaN layers with different Mg-concentrations were grown on SiC-substrates and Pt-contacts were processed. For those samples, we investigated the Mg-concentrations, verified by secondary ion mass spectroscopy (SIMS), the hole concentrations and mobilities in dependence of C(Mg), which we obtained from HALL-measurements, and the contact and sheet resistances, measured by circular TLM measurements.

The experiments showed that the optimum Mg-concentration for low contact resistances is higher than 2 1019 cm-3 which was found to provide a maximum hole concentration near 7 1017 cm-3. The influence of self-compensation in p-GaN in bulk and near interfaces will be discussed.
X-ray Photoemission Determination of the Surface Fermi Level Motion and Pinning on n- and p-GaN during the Formation of Au, Ni, and Ti Metal Contacts
Authors: Rickert Kimberly A. , Kim Jong Kyu , Lee Jong-Lam , Himpsel Franz J. , Ellis Arthur B. , Kuech T. F.
Published: April 2011

Synchrotron radiation-based x-ray photoemission spectroscopy was used to study the Fermi level position within the band gap for thin metal overlayers of Au, Ni, and Ti on n-GaN and p-GaN. The Fermi level position was determined with the measured Fermi edge of the metal on the sample in order to correct for the presence of non-equilibrium effects. There are two different behaviors observed for the three metals studied. For Au and Ti, the surface Fermi positions on n-GaN and p-GaN are roughly 0.5 eV apart within the band gap. For Ni, the n-GaN and p-GaN have a Schottky barrier that forms at the same place at the gap.
Structural Properties of GaN films grown by Molecular Beam Epitaxy on vicinal SiC(0001)
Authors: Lee C. D. , Feenstra R. M. , Shigiltchoff O. , Devatya R. P. , Choyke W. J.
Published: April 2011

Gallium nitride films are grown by plasma-assisted molecular beam epitaxy (MBE) on vicinal 6H-SiC(0001) substrates with [1 1 00] and [11 2 0] miscut directions. The hydrogen-etched substrates display straight, or chevron shaped steps respectively, and the same morphology is observed on the GaN films. X-ray rocking curves display substantially reduced width for films on the vicinal substrates compared to singular substrates, for the same Ga/N flux ratio used during growth.
Synthesis of oxygen-free nanosized InN by pulse discharge
Authors: Yang Wei-Dong , Cheah K.W. , Wang Pei-Nan , Li Fu-Ming
Published: April 2011

Oxygen-free nanosized InN was synthesized by pulse discharge. NH3/N2 mixture and bulk indium were used as nitrogen source and discharge cathode, respectively. Optical emission spectra show that N2 and NH3 were highly dissociated in the discharge and oxygen could be eliminated effectively. A comparison was made among samples produced in the discharge of N2, NH3 and N2/NH3, respectively. The XPS was used to determine the composition of the synthesized materials. Only In-N was produced when N2/NH3 was used as the discharge gas, whereas indium oxide was found in the case using N2 or NH3 as the discharge gas. The size of the produced InN was between 5 and 200 nm. A broad band with a blue shift of 150 nm from that of bulk InN was detected in the photoluminescence of the produced nanosized InN.
Micro-Raman Scattering From Hexagonal GaN, AlN, and AlxGa1-xN Grown on (111) Oriented Silicon: Stress Mapping of Cracks
Authors: Ramkumar C. , Prokofyeva T. , Seon M. , Holtz M. , Choi K. , Yun J. , Nikishin S. A. , Temkin H.
Published: April 2011

We report post-growth micro-Raman stress mapping of cracks in GaN, AlN, and AlxGa1-xN grown on (111) oriented silicon. Cracks with an average spacing of ~ 100 m are observed. These cracks are categorized into two types. The first type of crack propagates through the epilayer, and several microns deep into the substrate and is observed in all the samples investigated. The second type cracks epilayer only and is observed only in GaN. The micro-Raman stress mapping of the first type of crack shows that the epilayers are under biaxial tensile (< 0) stress and the silicon substrate is under compressive (> 0) stress far away from the cracks. The stress in the epilayers as well the substrate is found to relax from the equilibrium (far away from the cracks) value of –0.5 GPa (AlN), -0.16 GPa (GaN), -0.6 GPa (AlxGa1-xN) and 0.36 GPa (Si) as the crack position is approached. Partial relaxation is observed to occur over a range of 10 m m. At the crack position, the epilayers and the substrate are relaxed to nearly zero stress values. The stress mapping of the second type of crack reveals that the substrate is completely relaxed (stress is close o zero) far away from the cracks. At the crack position the GaN epilayer is partially relaxed from –0.2 GPa to –0.08 GPa, while the silicon substrate is seen to be under tensile stress of –0.39 GPa. The stress map of epilayers is well described by the distributed force model for both types of cracks. Furthermore, the calculated stress profiles of cracked and uncracked substrate using the above mentioned model are in excellent agreement with the experimental data.
400-nm Band AlGaInN-Based High Power Laser Diodes
Authors: Asano Takeharu , Takeya Motonobu , Tojyo Tsuyoshi , Ikeda Shinro , Mizuno Takashi , Ansai Shinichi , Goto Shu , Kijima Satoru , Hino Tomonori , Uchida Shiro , Ikeda Masao
Published: April 2011

High-power AlGaInN-based laser diodes (LDs) operating with high reliability in the 400-nm band have been successfully fabricated using a high-productivity process. Epitaxial lateral overgrowth (ELO) over a 10-m m region was employed to obtain a broad growth area with low dislocation density, and the thickness of the ELO-GaN layer was limited to approximately 5 m in order to minimize wafer bending. These techniques allow for the easy and reproducible alignment of the laser stripe on the region of low dislocation density. The insertion of a GaInN interlayer between the active layer and the AlGaN electron blocking layer was effective for reducing the strain between these two layers, resulting in homogeneous luminescence from the active layer and lower operating current. A mean time to failure of 15000 h under 30-mW continuous-wave operation at 60°C was realized as a direct result of the lower operating current. Productivity was remarkably improved by performing epitaxial growth on a 3-inch substrate. Highly uniform laser wafers were successfully fabricated by achieving minimal temperature variation (1000 ±7°C) over the 3-inch substrate. The resultant laser structures varied in thickness by only ±5%, and the photoluminescence wavelength was consistent within ±2.5 nm over the entire 3-inch substrate. The average threshold current of 550 LDs selected from a fourth wafer was 32.7 mA, with small standard deviation of 3.2 mA.
AlGaN/GaN based MOSHFETs with Different Gate Dielectrics and Treatments
Authors: Mistele D. , Rotter T. , Bougriouaa Z. , Moermanna I. , Röver K.S. , Seyboth M. , Schwegler V. , Stemmer J. , Fedler F. , Klausing H. , Semchinova O.K. , Aderhold J. , Graul J.
Published: April 2011

AlGaN/GaN based hetero field effect transistors (HFETs) were capped with different dielectrics, characterized, and tested for DC performance. As dielectrics we use SiO2 and photoelectrochemical (PEC) grown AlxGa2-xO3. Combination of this two dielectrics show best performance with respect to gate leakage current and controllability of the drain current ID. TheMOSHEFTs work also at positive gate voltages in accumulation, which is also demonstrated in a broad transconductance peak. The PEC oxidation shows low density of interface states Dit and the insulating properties depend strongly on the PEC conditions. Pre-treatments before the SiO2 deposition result in varying threshold voltages |Vth| and it seems that (NH4)Sx pre-treatment leaves the surface in best conditions. Comparison of Ti/Al and Ti/Al/Ti/Au as source/drain contacts for AlGaN/GaN HFETs are done and the annealing behavior of Ti/Al/Ti/Au is displayed resulting in contact resistance as low as RC = 2 &mm after annealing at 850°C in N2.
Electrical Properties of n-GaN/p-SiC and n-AlGaN/p-SiC Heterojunction Diodes
Authors: Luo B. , Kim J. , Mehandru R. , Ren F. , Lee K. P. , Pearton S.J. , Polyakov A.Y. , Smirnov N.B. , Govorkov A.V. , Kozhukhova E.A. , Osinsky A.V. , Norris P.E.
Published: April 2011

Properties of n-GaN/p-SiC and n-AlGaN/p-SiC heterojunctions prepared by HVPE on 4H SiC substrates were studied by means of C-V, C/G-T, C-f, I-V and DLTS. It is shown, in agrrement with earlier publications, that the GaN/p-SiC HJ is staggered type II with ΔEc=-0.4 eV andΔEv=0.6 eV. Whenchanging GaN for AlGaN with Al mole fraction of x=0.25-0.3 the band alignment becomes normal type I with ΔEc=0.2 eV andΔEv=0.6 eV. I-V characteristics of both heterojunctions bear evidence of strong tunneling via defect states, particularly centers with activation energy of 1.25 eV for GaN/4H SiC HJ. The tunneling was found to be more pronounced in the AlGaN/SiC HJs even though these HJs showed no evidence of formation of dark line defects at the interface, in contrast to GaN/SiC. DLTS measurements on both types of HJs revealed the presence of broad bands whose behavior is indicative of these bands being related to continuous states in the gap, most likely near the nitride/carbide interface.
Crystalline and Electrical Properties of AlInN/GaN and AlN/GaN superlattices on GaN Grown by Metalorganic Vapor Phase Epitaxy
Authors: Yamaguchi Shigeo , Iwamura Yasuo , Kosaki Masayoshi , Watanabe Yasuhiro , Mochizuki Shingo , Nakamura Tetsuya , Yukawa Yohei , Nitta Shugo , Kamiyama Satoshi , Amano Hiroshi , Akasaki Isamu
Published: April 2011

We have studied the crystalline and electrical properties of AlInN/GaN superlattices (SLs) and strained AlN/GaN SLs on GaN grown by metalorganic vapor phase epitaxy. A (0001) sapphire substrate was used. The SLs were grown using N2 carrier gas. X-ray analysis showed the eighth order satellite peak in AlInN/GaN SLs. Hall measurement showed an electron mobility of 946 cm2/Vs at highest (a sheet carrier density of 2.9x1012 cm-2) for AlInN/GaN 5SLs on GaN at 295K, and showed a value of 11432 cm2/Vs (1.99x1012cm-2) at 170K, and a value of 2610 cm2/Vs (3.38x1012cm-2) at 295K for AlN/GaN 10SLs on GaN.
Fabrication of Microstructures for Microphotonic Circuit
Authors: Chakraborty Subhasish , Hasko D G , Mears R J
Published: April 2011

We describe fabrication of sub-micron photonic bandgap structures on Si/SiO2l optical waveguide, which could be used atλ=1.54μm.
Erbium-Doped Barium Titanate Thin Film Waveguides For Integrated Optical Amplifiers
Authors: Teren Andrew R. , Kim Seong-Soo , Ho Seng-Tiong , Wessels Bruce W.
Published: April 2011

The factors affecting optical gain were studied for Er-doped BaTiO3 thin film waveguides. Er-doped BaTiO3 with dopant concentrations of 0.3 – 9 at.% was deposited by metal-organic chemical vapor deposition. The luminescence efficiency was maximized by optimizing the growth temperatureand erbium concentration as well as by post-deposition annealing. Stimulated emission was studied using the pump-probe technique over the spectral range of 1,520-1,550 nm. A maximum differential gain of 3 dB/cm wasmeasured at 1,540 nm in an 8 mm long, 8 μm wide ridge waveguide.
Analysis of Nucleation of Dislocation Loops from Stressed Surfaces Based on the Peierls-Nabarro Dislocation Model
Authors: Xu Guanshui
Published: April 2011

Nucleation of dislocation loops from stressed crystal surfaces is analyzed based on a variational boundary integral method in the Peierls-Nabarro framework. The stress dependent activation energies required to activate dislocation loops from their stable to unstable saddle point configurations are determined. Compared to previous analyses of this problem based on continuum elastic dislocation theory, the presented analysis provides more definitive solutions because it eliminates the uncertain core cutoff parameter by allowing for the existence of an extended dislocation core as the embryonic dislocation evolves. Moreover, the shape of the dislocation loop is solved by the variational principle instead of assumed to be semicircular as in previous analyses based on continuum elastic dislocation theory. It is noteworthy that the presented methodology can be readily used to study effects of surface inhomogeneities such as cracks and steps on dislocation nucleation.
Thermal Stability of Self-Supported Metallic Multilayered Thin Films
Authors: Misra A. , Kung H. , Hoagland R.G.
Published: April 2011

The morphological stability and strength retention following elevated temperature exposure or thermal cycling will be crucial in exploiting the extremely high strengths of nanolayered materials in advanced engineering applications. The effects of elevated temperature (≤800°C) vacuum annealing on the morphological stability and mechanical properties of sputter deposited Cu-Nb multilayers with 75 nm bilayer period are reported here. Even after 800°C/ 1 hour anneal, the continuity of the layered structure is maintained and the bilayer periods are unchanged. The in-plane grain sizes in both Cu and Nb coarsened but were anchored at grooved boundaries preventing further growth. For a constant bilayer period, the effect of increasing the in-plane grain size on the multilayer hardness is found to be insignificant. After annealing, the layers are observed to be offset by shear along a vertical plane at the triple point junctions that have equilibrium groove angles aligned in a zig-zag pattern. A new mechanism is proposed for the evolution of this “anchored” structure that is resistant to further morphological instability.
A New Type of Dislocation Mechanism in Ultrathin Copper Films
Authors: Balk T. John , Dehm Gerhard , Arzt Eduard
Published: April 2011

In this study of thin film plasticity, the relationship between thermomechanical behavior and dislocation motion has been investigated in copper constrained by a silicon substrate. The stress-temperature behavior as determined from wafer curvature experiments has been directly compared to deformation microstructures observed during in situ thermal cycling of plan-view specimens in the transmission electron microscope. The flow stress of copper films with thicknesses ranging from 100 nm to 400 nm was found to be constant, indicating that strengthening mechanisms may be saturated in this thickness regime. Moreover, unexpected dislocation glide on a plane parallel to the film surface, which should experience no resolved shear stress, provides potential evidence for the occurrence of constrained diffusional creep in a 270 nm film.
Bulge Test Characterization of Static Softening and Dynamic Instabilities in Foils of an Al-Based Alloy
Authors: Cieslar Miroslav , Karimi Ayatollah , Martin Jean-Luc
Published: April 2011

Effects of recovery and recrystallization on subsequent stress – strain behavior were studied in foils of non-age hardenable Al based alloy. A bulge testing device for mechanical testing of thin free standing films and foils, enabling the detection of large strains, was employed. The bulge tester was used at RT both for the predeformation of the foil in the biaxial mode and also for the study of softening after subsequent one-step annealing. Three stages of strength drop as a function of the annealing temperature were observed between RT and 590°C. The contribution of different annealing processes to the softening was established using transmission electron microscopy. It was found that below 200°C only redistribution of dislocations inside dislocation cells and refinement of the cell structure occurred. Between 200°C and 380°C the formation of a subgrain structure was observed. The softening process is terminated as partial recrystallization takes place at higher annealing temperatures. Dynamic interaction of solutes with dislocations was revealed during prestraining as well as poststraining of foils. This effect resulted in the appearance of a negative strain rate sensitivity and dynamic instabilities after appropriate prestraining and annealing conditions. The above results show that bulge testing of Al foils allows to study the successive annealing stages by measuring the most important macroscopic parameters of plastic deformation (yield stress, work hardening rate, ductility, strain rate sensitivity, etc.). These stages could be related to the microstructure evolution.
Gold/Niobium Thin Film Metallizations for GaAs Devices and Circuits
Authors: Esser Robert , Christou Aris
Published: April 2011

A refractory metallization of Au/Nb is proposed for use in first level metallization of GaAs devices. The diffusion and reaction kinetics are explored using sheet resistance measurements, along with X-ray diffraction and Auger electron spectroscopy depth profiling. The interdiffusion coefficients are reported. Diodes are fabricated using Nb/Au metallization and characterized
The Effects of Copper on the Interfacial Failure of Gold Films
Authors: Moody N. R. , Adams D. P. , Cordill M. J. , Yang N. , Bahr D. F.
Published: April 2011

Nanoindentation test techniques were combined with deposition of highly stressed overlayers to study the interfacial fracture susceptibility of gold-on-copper and gold-2w/o-copper alloy films. The gold-on-copper film blistered readily following deposition of stressed tungsten overlayers. Additional stress from nanoindentation was required to trigger delamination and blister formation in the gold-copper alloy film. Fracture energies were then determined using mechanics-based models. The results show that the gold-copper alloy exhibited higher fracture energies than the gold-on-copper films. This increase scaled with film strength suggesting that the higher measured fracture energies in the gold-copper alloy film were due to solid solution hardening.
Interfacial Adhesion of Cu to Self-Assembled Monolayers on SiO2
Authors: Cui G. , Lane M. , Vijayamohanan K. , Ramanath G.
Published: April 2011

As the critical feature size in microelectronic devices continues to decrease below 100 nm, new barrier materials of > 5 nm thickness are required. Recently we have shown that self-assembled monolayers (SAMs) are attractive candidates that inhibit Cu diffusion into SiO2. For SAMs to be used as barriers in real applications, however, they must also promote adhesion at the Cu/dielectric interfaces. Here, we report preliminary quantitative measurements of interfacial adhesion energy and chemical binding energy of Cu/SiO2 interfaces treated with nitrogen-terminated SAMs. Amine-containing SAMs show a ~10% higher adhesion energy with Cu, while interfaces with Cu-pyridine bonds actually show degraded adhesion, when compared with that of the reference Cu/SiN interface. However, X-ray photoelectron spectroscopy (XPS) measurements show that Cu-pyridine and Cu-amine interactions have a factor-of-four higher binding energy than that of Cu-N bonds at Cu/SiN interfaces. The lack of correlation between adhesion and chemical binding energies is most likely due to incomplete coverage of SAMs.
Effects of Film Thickness on the Yielding Behavior of Polycrystalline Gold Films
Authors: Espinosa H.D. , Prorok B.C.
Published: April 2011

A Membrane Deflection Experiment was used to test the mechanical response of freestanding thin film gold specimens. We present stress-strain curves obtained on films 0.3, 0.5 and 1.0 μm thick. Elastic modulus was consistently measured in the range of 53-55 GPa. Several size effects on the mecha nical properties were observed including yield stress variations with membrane width and film thickness. It was observed that thickness plays a key role in deformation behavior with a major transition in the material inelastic response occurring between a thickness of 0.5 and 1.0 μm. The size effects here reported are the first of their kind in the sense that the measurements were performed under a macroscopically homogeneous axial deformation, i.e., in the absence of macroscopic deformation gradients.
Simulation of Mechanisms in Strained Silicon Germanium Epitaxy
Authors: Wagner Richard J. , Gulari Erdogan
Published: April 2011

Growth of strained semiconductors can lead to self-assembly of interesting structures such as quantum dots. Simulation of this growth requires an accurate and efficient model for the interactions of lattice mismatched materials. We present a potential for calculating the bond energies in a diamond-like crystal of silicon and germanium. With this potential we predict the strain profiles in an embedded quantum dot and discuss the mechanisms of island formation.
Stress Generation and Relaxation during Film Heteroepitaxy on a Compliant Substrate with a Viscoelastic Glass Interlayer
Authors: Feng Zhaohua , Lovell Edward G. , Engelstad Roxann L. , Moran Peter D. , Kuech Thomas F.
Published: April 2011

Lattice mismatch strain between films and substrates causes stresses in each and degrades the film quality. Compliant substrates can decrease the stresses and dislocation density in the film. A particular type of compliant substrate, which consists of a thin template, a handle wafer and a glass interlayer, is discussed here. Three-dimensional axisymmetric finite element models were developed to simulate the film-substrate structure and analyze stress generation and relaxation. The materials of film and template were considered as elastic but the glass interlayer was viscoelastic at the film growth temperature. Factors affecting stress generation and relaxation are reported.
Shape Transitions of Self-Assembled Ge Islands on Si (001)
Authors: Rastelli Armando , Kummer Matthias , von Känel Hans
Published: April 2011

Coherently strained Ge islands were grown at a substrate temperature of 550°C by magnetron sputter epitaxy on Si (001) and studied by scanning tunnelling microscopy (STM). The shape changes induced by exposure to a Si-flux at 450°C were investigated as a function of the Sicoverage. During Si-capping, multifaceted domes were found to flatten and to transform into {105}-faceted pyramids and subsequently into stepped mounds through intermediate shapes. The observed sequence of morphological changes is induced by Si-Ge intermixing and is shown to be the inverse of that occurring during Ge or Si1-xGex growth on Si (001). The results are interpreted with a model in which the stable shape of an island mainly depends on its volume and composition.
Compositionally Modulated Structures Studied by in Situ Scanning Tunneling Microscopy
Authors: Pearson Chris A. , Dorin Catalina , Millunchick Joanna Mirecki , Chen Yunqing , Orr Brad G.
Published: April 2011

Two different short period superlattice (SPS) structures with nominally equivalent lattice mismatch, InAs/AlAs and InAs/GaAs are examined using in situ scanning tunneling microscopy (STM). Depending upon the growth conditions, the composition of the InAs/AlAs SPS structure can be either homogeneous or modulated in the lateral direction. Distinct periodic structures are clearly visible in images of the modulated SPS while no periodicity is observed in the homogeneous SPS. For a 30 period SPS consisting of 2 monolayers of AlAs and 2 monolayers of InAs we observe structures 20 nm in size with an average spacing of ∼25 nm in orthogonal directions, which is approximately the same length scale as the composition modulation. Despite the nominally equivalent lattice mismatch, the InAs/GaAs SPS structures are quite different. Some degree of modulation is always observed in these structures. Homogeneous structures are not observed. For the modulated SPS structures, scanning tunneling spectroscopy can be used to characterize the chemical profile.
Computer Simulation on the Relations between the Porosity and the Microwave Properties in Dielectric Ceramics
Authors: Park Jae-Hwan , Park Jae-Gwan , Kim Yoonho
Published: April 2011

Effects of porosity on the microwave properties in dielectric ceramics were studied by the electromagnetic simulation. Scattering matrix S21 obtained from the network analyzer was compared to the S21 obtained from the simulation. From electric field distribution, the dominant resonant TE01 mode could be easily determined. The effects of the porosity inside the dielectrics on the microwave properties were investigated. Quality factor decreased with the porosity and the pore size.
Determination of Surface Space Charge Density on Semiconductor from Displacement Current-Voltage Curve using a Scanning Vibrating Probe
Authors: Majima Yutaka , Masuda Tomohiko , Uehara Setsuri , Okuda Atsushi , Iwamoto Mitsumasa
Published: April 2011

A determination method of the local surface space charge density in thin films on semiconductor surfaces is described. In this method, the displacement current and the tunneling current flow periodically in accordance with the perpendicular vibration of the probe of the scanning tunneling microscopy (STM). The external circuit current is separated into the mean displacement current and the mean tunneling current by using the two-phase lock-in amplifier. The displacement current-probe voltage curve of a p-type silicon with a native oxide layer was measured in a vacuum (>3×10−9 torr). The displacement current-voltage curve has been analyzed taking into account surface space charge. The theoretical displacement current-voltage curve has been fitted to the measurement in order to obtain both the local surface space charge density and the surface potential difference of thin films, and the theory is in good agreement with the measurement in both voltage regions where the majority carriers are accumulated and depleted. We also demonstrate a simple determination method of both the surface space charge density and the surface potential difference of thin films from flat-band conditions in the displacement current-voltage curve.
Conductance-transient three-dimensional profiling of disordered induced gap states on metal-insulator-semiconductor structures
Authors: Castán H. , Dueñas S. , Barbolla J. , Mártil I. , González-Díaz G.
Published: April 2011

As it has been shown elsewhere, conductance transient measurements provide quantitative information about the disordered induced gap states (DIGS) in metal-insulator-semiconductor (MIS) structures. In this work we report for the first time the DIGS spatial and energetical distribution obtained by recording conductance transients at several temperatures (ranging from 77 to 300 K) and several frequencies (ranging from 100 Hz to 200 KHz). These measurements allow us to obtain three-dimensional defect maps of Al/SiNx:H/InP structures browsing ranges of 0.5 eV in energy and 40 Å in depth. Our results show that this technique is a very useful tool for the electrical characterization of MIS structures and reveals itself as very valuable in the III-V semiconductor-field-effect transistor scenario.
Application of a Buffer Layer for the Dielectric Measurement of Thin Polymer Films
Authors: Chiang C. K. , Sakai Wataru
Published: April 2011

A buffered dielectric measurement method is described. We added a thin buffer polymer layer to a polymer film before depositing aluminum electrodes. This is a modification to conventional parallel plate dielectric constant measurement method. It still has well-defined geometric factor for determining the dielectric constant. We designed the buffer layer using a simple RC model. It was determined that the buffer layer should be a high dielectric constant polymer. Two high dielectric constant polymers were selected to be buffer layers. Layered samples with structures ABA and ABC were discussed, where A is the buffer layer. We show that the method not only provides a way to preserve the structure of special polymer films, but also is able to adjust its electrical characterization to a convenient level.
Development of Scanning Microwave Microscope for High-Throughput Characterization of Combinatorial Dielectric Thin Film
Authors: Okazaki Noriaki , Ahmet Parhat , Chikyow Toyohiro , Odagawa Hiroyuki , Cho Yasuo , Fukumura Tomoteru , Kawasaki Masashi , Ohtani Makoto , Koinuma Hideomi , Hasegawa Tetsuya
Published: April 2011

A scanning microwave microscope (Sm M) for high-throughput characterization of combinatorial dielectric materials has been developed using a lumped constant resonator probe. The probe consists of a microwave oscillator module equipped with a thin conducting needle and an outer conductor ring, which detects the dielectric constant of the sample just beneath the needle as a frequency shift of the resonator. The quantitative analysis of the dielectric constant for the bulk and the thin-film samples was carried out based on the measurement of gap-length dependence of the frequency shift. The analysis method was successfully applied to the characterization of composition-spread BaxSr1-xTiO3 thin film sample. The evaluation of far-field contribution to the frequency shift was found to be crucial for the accurate determination of dielectric constant especially in the characterization of combinatorial thin films.
Computer-Controlled Fabrication of Concentric Multilayer Fresnel Zone Plate for Synchrotron Radiation Hard X-Ray
Authors: Yasumoto Masato , Tamura Shigeharu , Kamijo Nagao , Suzuki Yoshio , Awaji Mitsuhiro , Takeuchi Akihisa , Takano Hidekazu , Kohmura Yoshiki , Handa Katsumi
Published: April 2011

A Fresnel zone plate (FZP) is a micro-focusing optical element of synchrotron radiation hard X-ray. Our developed multilayer FZP produced a sub-micron size spot at the hard X-ray energy and the focusing efficiency is about 25%. However the higher efficiency FZP is required to utilize the sub-micron probe in microscopy, diffraction and imaging applications. A kinoform FZP composed of gradient refractive index phase zones has the advantage of a high focusing efficiency compared to the conventional multilayer FZP. In order to fabricate the kinoform FZP, we developed a computer-control system. The developed computer-control system automatically operates whole combinatorial deposition processes of the kinoform FZP by inputting the optical parameters of the FZP (focal length, X-ray wavelength, number of zones, zone materials).
Artificial Intelligence In Materials Science: Application to Molecular and Particulate Simulations
Authors: Maguire John F. , Benedict Mark , Woodcock Leslie V. , LeClair Steven R.
Published: April 2011

We illustrate how emerging methods in artificial intelligence (AI) may be useful in materials science. Historically, these methods were developed in the area of materials process control and, more recently, in the nascent field of materials discovery. However, machine intelligence is of much broader import and our primary objective here is to illustrate how such methods may be used to circumvent some serious roadblocks in the computer simulation of a significant class of computationally hard problems in materials science. This is illustrated by a new approach to solving the dynamics of the N-body problem for large numbers of objects of essentially arbitrarily complex geometry or interaction potential. The approach, based on a particulate artificial neural net dynamics algorithm (PANNDA) is more than two orders of magnitude faster than existing methods when applied to large systems and is only marginally slower (∼10%) than the theoretical lower limiting case of hard spheres. In this method an artificial neural net is trained to predict accurately the time to next collision for binary encounters spanning the Hilbert space of relative positions, orientations and momenta (linear and angular). This approach, which can be extended to soft complex systems, enables construction of exact, albeit numerical, models for the thermodynamic, transport and non-equilibrium properties of very large ensembles of hard or soft objects of arbitrarily complex shape or interaction potential. Our results open up the possibility of immediate application to an usually wide spectrum of contemporary computationally intractable “hard” problems ranging from granular materials with asperities through inclusion of complex many-body terms in the intermolecular interaction in molecular dynamics calculations of complex fluids and polymers.
Heuristic Methods for Finding Ground-states of Ising Models
Authors: Lassalle Hugues J. , Wille Luc T.
Published: April 2011

We describe rhe application of simulated annealing and genetic algorithms to determine the ground-state of various classes of Lsing models. This problem is relevant to finding equilibrium contigurations(at zero Kelvin)of adsorbed monolayers, multi-component alloys, and magnetic systems. Because of the presence of metastable configurations (local minima) the detection of the ground-state (global minimum) is a non-trivial problem, especially in the case of complex interactions or frustrated systems. The speed of convergends analyzed for various model systems.
Simulation of Crystal-Melt Interfaces for a System of Binary Hard Spheres
Authors: Sibug-Aga Rachel , Laird Brian B.
Published: April 2011

Crystal-melt interfaces of binary hard spheres are investigated using molecular-dynamics simulation. For a diameter ratio α=0.414, two crystal phases coexisting with the fluid are possible, depending on the pressure. At low pressures, the liquid coexists with a pure fcc crystal of the larger particle, while at high pressures a 1:1 binary crystal of “NaCl” type is the coexisting phase. For both of these systems, we study the structural and dynamical changes as the interface is traversed from bulk crystal to bulk fluid through the calculation of density and diffusion coefficient profiles. It is observed that the total width of the interfacial region is narrower in the “NaCl”/binary fluid interface than in the corresponding lower pressure fcc/binary fluid system.
Applications of Bio-Composites in Industrial Products
Authors: Kandachar Prabhu , Brouwer Rik
Published: April 2011

Available as agricultural resources in many countries, natural fibers, such as flax, hemp, kenaf, exhibit mechanical properties comparable to those of synthetic fibers like glass. But they are lighter, biodegradable, and are often claimed to be less expensive. Composites with these natural fibers have the potential to be attractive alternative to synthetic fiber composites. The natural fibers, however, exhibit more scatter in their properties, are thermally less stable and are sensitive to moisture absorption. The choice of matrix to reinforce with these fibers therefore becomes critical.

Currently, synthetic non-biodegradable polymers, such as polypropylene, polyester, etc., are being explored as matrix materials, for applications in sectors like automobiles and buildings. Biodegradable polymers, if made available in sufficient quantities at affordable prices, pave way for bio-composites in future. With both matrix and fibers being biodegradable, bio-composites become attractive candidates from the environment point of view.

Extensive and reliable property data on natural fiber composites and/or on bio-composites, are still lacking, making product design with these materials rather tedious. Once the database is available, design & manufacture of products with natural fiber composites and biocomposites offer several opportunities and challenges.