bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlation between Surface Charge Accumulation and Excitation Intensity Dependent Red-Shifted Micro-Photoluminescence of Si-Implanted Quartz with Embedded Si Nanocrystals
Authors: Lin Chun-Jung , Yu Kuo-Cheng , Kuo Hao-Chung , Ou-yang Miao-Jia , Lin Gong-Ru
Published: April 2011

The excitation intensity dependency of nanocrystallite Si (nc-Si) related micro-photoluminescence (μ-PL) from the multi-recipe Si-implanted quartz is characterized. The μ-PL at 724 nm contributed by nc-Si with 3-4 nm diameter is maximized after annealing at 1100°C for 3 hours. By increasing the excitation intensity from 10 kW/cm2 to 300 kW/cm2, the μ-PLs of 1-hr and 3-hr annealed quartz significantly red-shift from 723 nm to 725 nm and from 724 nm to 735 nm, respectively. This can be explained by the anomalous quantum stark effect due to a surface electric field oriented from photo-ionized nc-Si dots to quartz surface. After 1-hr illumination at power of 300 kW/cm2, the μ-PL peak wavelength of 3-hr annealed sample is further red-shifted by 2.5 nm. By measuring the accumulated surface charges built up during optical excitation process, the correlation between excitation-intensity dependent PL wavelength red-shift and the photo-ionized nc-Si surface electric-field related quantum stark effect is primarily elucidated.
Buffer-layer Effect on Mixed-Phase Cells Studied by Micro-Raman and Photoluminescence Spectroscopy
Authors: Hilchey Andrea , Lawyer Chris , Wang Keda , Han Daxing , Yan Baojie , Yue Guozhen , Yang Jeffrey , Guha Subhendu
Published: April 2011

We use micro-Raman and photoluminescence (PL) spectroscopy to study the effects of an a-Si:H buffer layer at the i/p interface of the mixed-phase silicon solar cells. We find that the signature of the crystalline 520 cm−1 mode still appears on the Raman spectrum for the cells with a 100 Å thick a-Si:H buffer layer; but it completely disappears for cells with a 500 Å thick a-Si:H buffer layer. At 80 K, the PL spectral lineshape reflects the features of the electronic states in the band tails. The characteristics of the PL spectra of the mixed-phase cells are a narrower main band than the standard a-Si:H band and an extra low energy band from the grain boundary region. As the thickness of the a-Si:H buffer layer increases, the PL main band becomes broader, and the low energy band is depressed. We find that, after light soaking, the PL main band is slightly broadened for the cells with no a-Si:H buffer layer, almost no change for the cells with a 100 Å thick buffer layer, and a remarkable decrease in total PL intensity for the cells with a 500 Å thick buffer layer. In addition, the PL intensity of the defect band increases after light soaking for the cells with a 500 Å thick buffer layer, where light-induced defect generation in the a-Si:H buffer layer masks the changes in the mixed-phase intrinsic layer. The Raman and PL results are consistent with previous observations of the effect of an a-Si:H buffer layer on the performance and metastability against light soaking for mixed-phase solar cells.
Influence of the Distribution of Tail States in a-Si:H on the Field Dependence of Carrier Drift Mobilities
Authors: Brinza Monica , Emelianova Evguenia V. , Stesmans André , Adriaenssens Guy J.
Published: April 2011

Exponential distributions of tail states have been able, within the framework of a multiple-trapping transport model, to account rather well for the time-of-flight photoconductivity transients that are measured with ‘standard’ a-Si:H, i.e. material prepared by plasma-enhanced chemical vapor deposition at ∼250°C. A field-dependent carrier mobility in the dispersive transport regime is part of the observations. However, samples prepared in an expanding thermal plasma, although still exhibiting the dispersive transients, fail to show this field dependence. The presence of a Gaussian component in the density of valence-band tail states can account for such behavior for the hole transients. Nanoscale ordered inclusions in the amorphous matrix are thought to be responsible for the Gaussian density of states contribution.
Amorphous Silicon based Solar Cell Technologies: Status, Challenges, and Opportunities
Authors: Arya Rajeewa R.
Published: April 2011

Advances in amorphous silicon solar cell and module development over the past two decades has led to widespread commercial application in consumer and building integrated photovoltaic applications (BIPV). The technology has taken two pathways: (i) superstrate and (ii) substrate. Both pathways have unique advantages over crystalline modules and have demonstrated promising stability and reliability with continuous improvement in performance. Multi-junction modules with amorphous and microcrystalline silicon have demonstrated initial conversion efficiencies in the range of 13%-13.5%.
Hole Drift-Mobility Measurements and Multiple-Trapping in Microcrystalline Silicon
Authors: Dylla T. , Finger F. , Schiff E. A.
Published: April 2011

We present photocarrier time-of-flight measurements of the hole drift-mobility in microcrystalline silicon samples with a high crystalline volume fraction; typical room-temperature values are about 1 cm2/Vs. Temperature-dependent measurements are consistent with the model of multiple-trapping in an exponential bandtail. While this model has often been applied to amorphous silicon, its success for predominantly crystalline samples is unexpected. The valence bandtail width is 31 meV, which is about 10-20 meV smaller than values reported a-Si:H, and presumably reflects the greater order in the microcrystalline material. The hole band-mobility is about 1 cm2/Vs – essentially the same magnitude as has been reported for electrons and for holes in amorphous silicon, and suggesting that this magnitude is a basic characteristic of mobility-edges, at least in silicon-based materials. The attempt-frequency is about 109 s-1; this value is substantially smaller than the values 1011 - 1012 s-1 typically reported holes in amorphous silicon, but the physical significance of the parameter remains obscure.
Enhanced Surface Diffusion in Low-temperature a-Si:H Processing
Authors: Dalakos George T. , Plawsky Joel L. , Persans Peter D.
Published: April 2011

Glow discharge amorphous hydrogenated silicon (a-Si:H) prepared at near room temperature typically results in an inhomogeneous morphology that is undesirable for a number of thin film applications. The most commonly observed features of this include columnar morphology and surface roughness. This usually results from anodic deposition, where substrates are placed on the grounded electrode. We have discovered that placing substrates on the RF-powered electrode (referred to as cathodic deposition) offers a much wider processing range for homogenous growth than anodic growth. We have also found that the magnitude of the surface roughness and the bulk void fraction of both anodic and cathodic a-Si:H thin films processed at low-temperatures is proportional to ∼D/F, where D is the surface diffusivity and F, the adatom flux, though anodic and cathodic deposition affect these global parameters differently. Surface processes unique to cathodic deposition can enhance adatom surface diffusion, while diffusion during anodic deposition is fixed and cannot attain homogeneous growth at high adatom fluxes. Processing a-Si:H on the cathode, associated with enhanced adatom surface diffusion, allows for homogeneous growth even at high deposition rates that has benefits for a number of applications.
Non-contact Electrical Measurements of Sheet Resistance and Leakage Current Density for Ultra-shallow (and other) Junctions
Authors: Faifer Vladimir N. , Current Michael I. , Walecki Wojtek , Souchkov Vitali , Mikhaylov Georgy , Van Phuc , Wong Tim , Nguyen Tan , Lu Jiansong , Lau S.H. , Koo Ann
Published: April 2011

A novel, non-contact method for determination of ultra-shallow junction sheet resistance and leakage current density has been developed based on monitoring the dynamics of photo-generated carriers by means of spatially separated capacitive probes. At light modulation frequencies of about 100 kHz, spatially resolved surface voltage signals give a direct measure of the junction sheet resistance, independent of the junction depth. At lower light modulation frequencies, the junction leakage current density is determined. Combining capacitive monitoring of modulated photo-generated free carriers with a precision wafer motion stage allows for rapid acquisition of sheet resistance and leakage data for efficient wafer-scale mapping applications.
Electrical profiles of ultra-low energy antimony implants in silicon
Authors: Alzanki T. , Gwilliam R. , Emerson N. , Sealy B. J. , Collart E.
Published: April 2011

A novel Differential Hall Effect technique is used to obtain doping profiles of 2keV implants of antimony in <100> silicon at a depth resolution of 2nm. The electrical profiles are compared with atomic profiles determined from Secondary Ion Mass Spectroscopy (SIMS) measurements. We demonstrate good consistency between the two profiling techniques and confirm that the SIMS profiles can be used to identify the junction depth. The profiles show that antimony does not diffuse significantly for annealing temperatures below 800°C.
Atomistic Modeling and Simulation of Impurity Atmosphere in Silicon and Edge Dislocation Locking Effects
Authors: Karoui A.
Published: April 2011

A theoretical study of edge dislocation locking by impurities in silicon is presented. Three groups of impurities are considered: (i) light atoms O, N, and C., (ii) large atoms Ga, and Ge, and (iii) small dopant atoms B, P, and Al. Based on impurity size effect model, these three groups produce distinct different dislocation locking effects. Atoms from the first group strongly bind with edge dislocations. The O, N, and C atmospheres are similar, with a slightly stronger occupancy probability for O and N in the vicinity of the dislocation core. For the second group, Ge loosely binds to dislocation and resists at most 1/3 of the separation shear stress that the first group can withstand. Germanium has only a small chance to reach the dislocation core. The third impurity group does not resist shear any separation stress from edge dislocations. Moreover, B and P atoms can not be trapped at all by edge dislocations. At a local atomic fraction of 10−4, edge dislocation-impurity binding energy varies from 0.008 eV/Å for P to 1.7 eV/Å for N and 1. 8 eV/Å for O. In addition, using molecular mechanics on system of 34552 atoms the self-energy of an edge dislocation was calculated and found equal to 156 meV/Å.
Optimization of Dielectric Cap Adhesion to Ultra-Low-k Dielectrics
Authors: Spencer Greg , Soyemi Alfred , Junker Kurt , Vires Jason , Turner Michael , Kirksey Stuart , Sieloff David , Ramani Narayanan
Published: April 2011

In this work, the adhesion of CVD dielectric caps to ULK MSQ spin-on dielectric materials with k values of 2.2 and 2.0, and a ULK CVD material with a k value of 2.7 is presented. A substantial improvement in cap adhesion to both the k2.2 ULK MSQ and the k2.7 ULK CVD material is demonstrated. The improvement is obtained using a low-k CVD glue material between the ULK dielectric and the subsequent cap material and/or by optimizing the CVD cap film deposition. Four-point bend measurement of adhesion strength is used to quantify the improvement in interface adhesion. The improvement in CVD cap adhesion is demonstrated to be strongly dependent upon both the glue layer film and the cap deposition conditions. While optimization of the CVD cap materials results in adequate adhesion for the k2.2 ULK MSQ, these improvements are demonstrated not to extend to the k2.0 ULK MSQ film.
Comparative Studies of Ultra Low-κ Porous Silica Films with 2-D Hexagonal and Disordered Pore Structures
Authors: Fujii Nobutoshi , Yamada Kazuhiro , Oku Yoshiaki , Hata Nobuhiro , Seino Yutaka , Negoro Chie , Kikkawa Takamaro
Published: April 2011

Periodic 2-dimensional (2-D) hexagonal and the disordered pore structure silica films have been developed using nonionic surfactants as the templates. The pore structure was controlled by the static electrical interaction between the micelle of the surfactant and the silica oligomer. No X-ray diffraction peaks were observed for the disordered mesoporous silica films, while the pore diameters of 2.0-4.0 nm could be measured by small angle X-ray scattering spectroscopy. By comparing the properties of the 2-D hexagonal and the disordered porous silica films which have the same porosity, it is found that the disordered porous silica film has advantages in terms of the dielectric constant and Young's modulus as well as the hardness. The disordered porous silica film is more suitable for the interlayer dielectrics for ULSI.
Effect of Mode-Mixity and Porosity on Interfacial Fracture of Low-k Dielectrics
Authors: Merrill Caroline C. , Ho Paul S.
Published: April 2011

In this study, we developed a system allowing interfacial adhesion measurements as a function of mode-mixity, from pure tension to pure shear. Results show that the debonding energy increases, by a factor of 3 to 10, as the amount of shear stress increases, approaching mode II conditions. For low k dielectrics, the debonding energy was found to decrease with increasing porosity and increase with increasing plasticity. The crack propagation is also dependent on mode-mixity.
Effect of Aqueous Solution Chemistry on the Accelerated Cracking of Lithographically Patterned Arrays of Copper and Nanoporous Thin-Films
Authors: Guyer E. P. , Dauskardt R. H.
Published: April 2011

The effect of moisture and aqueous solution chemistry on the rate of crack growth in lithographically patterned arrays of copper and low dielectric constant (LKD) materials is reported. Crack growth in the direction orthogonal to the features is demonstrated to fail at significantly increased loads compared to parallel cracking. Decreasing feature width is also shown to increase the structures resistance to fracture, particularly at reduced loads. The chemical interactions and mechanisms of energy dissipation are discussed.
Effect of Surface Chemistry on the diffusion of Copper in nanoporous dielectrics
Authors: Rodriguez Oscar , Cho Woojin , Saxena Ravi , Achanta Ravi , Gill William N. , Plawsky Oel L
Published: April 2011

This work is aimed at understanding the nature of the interactions between metal interconnects and nanoporous dielectrics in integrated circuits. Electrical testing of MIS capacitors is used to assess Cu diffusion and charge injection in the dielectric in the presence of an electric field. We have found that surface modification of nanoporous silica reveals the importance of chemically bound or adsorbed water species in the dielectric and how they trigger metal diffusion. We propose that a combination of moisture-related species in the dielectric and interfacial oxygen oxidize Cu. The copper oxide acts as a source for Cu ions available for diffusion. A quantitative analysis of Cu drift in nanoporous dielectrics that shows the importance of surface chemistry is presented and the mechanism of metal diffusion and charge injection in nanoporous dielectrics is discussed.
Mortality Dependence of Cu Dual Damascene Interconnects on Adjacent Segment
Authors: Chang C.W. , Gan C.L. , Thompson C.V. , Pey K.L. , Choi W.K. , Hwang N.
Published: April 2011

Three terminal ‘dotted-I’ interconnect structures, with vias at both ends and an additional via in the middle, were tested under a variety of test conditions. Failures (mortalities) were observed even when segments were tested under conditions that would not have led to failure in two-terminal structures. Mortalities were found in right segments with jL values as low as 1250 A/cm, which is lower when compared to the immortality condition (jL)cr of 3700 A/cm reported for similar via-terminated structures. Moreover, we found that the mortality of a dotted-I segment is dependent on the direction and magnitude of the current in the adjacent segment. These results suggest that there is not a definite value of the jL product that defines true immortality in individual segments that are part of an interconnect tree. More importantly, the critical jL value for a single segment of Cu interconnects may be reduced or increased by an adjacent segment. Therefore independently determined (jL)cr values cannot be directly applied to interconnects with branched segments, but rather the magnitude as well as the direction of the current flow in the adjoining segments must be taken into consideration in evaluating the immortality of interconnect segments in an interconnect network.
Polymer Based Photodetectors
Authors: Narayan K. S. , Kabra D. , Dutta S.
Published: April 2011

Recent developments in our laboratory related to polymer-based light sensors are reviewed. The inherent processibility of the active polymer medium is utilized in the implementation of different designs for the opto-electronic applications. The utility of these devices as sensitive photodetectors, image sensors and position sensitive detectors is demonstrated. The schottky-type layer formation at interfaces of polymers such as polyalkylthiophenes and aluminum accompanied by the enhanced photo-induced charge separation due to high local electric field is tapped for some of these device structures. The sensitivity of polymer-based field effect transistors to light also provides a convenient lateral geometry for efficient optical-coupling and control of the transistor state. The range of these polymer-detectors available with the option of operating in the diode and transistor modes should be an attractive feature for many potential applications.
Nano-Scale Devices Embedded in Self-Supporting Polymer Foils
Authors: Könenkamp R. , Chen J. , Klaumuenzer S. , Engelhardt R.
Published: April 2011

Self-supporting polymer foils of several micrometer thickness have been structured using irradiation by fast heavy ions and subsequent etching. Very deep, well defined nanostructures can be prepared in this way. These can then be used as templates for semiconductor deposition and the fabrication of robust embedded electronic devices. Electron microscopy shows void- filling poly-crystalline growth in the polymer films. Electrical experiments show low sensitivity when mechanical forces are exerted on the foil, suggesting that the embedded nano- devices can be used as reliable sensors in applications with considerable bending of the foils. Two different transistor arrangements will be discussed.
Compositional control and structural properties of ZnSe1-xTex epitaxial films on lattice- matched InGaAs/InP (001) by photoassisted Metal Organic Vapor Phase Expitaxy (MOVPE)
Authors: Yarlagadda Bhanu Prakash , Rodriguez Angel , Li Peng , Ayers John , Jain Faquir
Published: April 2011

We report the growth of ZnSe1−xTex (x < 0.9) epitaxial layers using photoassisted Metal- Organic Vapor Phase Epitaxy (MOVPE) on lattice-matched InGaAs/InP (001) substrates. Ternary compositional control was studied as a function of the gas phase composition, growth temperature (360°C – 400°C) and irradiation (12 mW/cm2 – 48mW/cm2). Compositional and structural data were obtained by x-ray rocking curves from 004 and 044 reflections. Lower growth temperatures increased the relative tellurium incorporation but at the expense of the growth rate.
Dynamics of grain boundary motion at the atomic level
Authors: Merkle K. L. , Thompson L. J. , Phillipp F.
Published: April 2011

Grain boundaries (GBs) in polycrystalline materials play a pivotal role in controlling their mechanical and physical behavior. High-resolution electron microscopy (HREM) was used to study thermally activated GB migration in thin films of Al and Au at elevated temperatures (T > 0.5 Tm). Grain boundary engineering via epitaxial templating allowed the manufacture of well-defined grain and interfacial geometries. These techniques enabled the observation of tilt, but also twist and general GBs at atomic resolution in-situ at high temperatures. Surface-energy driven GB migration occurred in general GBs, whereas tilt GB motion was curvature driven. Digital analysis of HREM video recordings have given considerable insight in the dynamics of GB motion at elevated temperatures. It is not surprising that the complex and diverse migration mechanisms depend on GB geometry as well as on interatomic interactions. The results provide, among others, direct evidence for collective effects by concerted atomic shuffles, ledge propagation in (113) symmetric tilt GBs, and motions of triple junctions at elevated temperatures.
In-Situ Investigation of Grain Boundary Mobility and Character in Aluminum Alloys in the Presence of a Stored Energy Driving Force
Authors: Taheri Mitra L. , Rollett Anthony D. , Weiland Hasso
Published: April 2011

This paper investigates the effect of solute in Al alloys on grain boundary character and mobility based on experiments in which individual boundaries migrate under a stored energy driving pressure acquired from prior plastic strain; among those studied are Zr, Fe and Si. A compensation effect is noted for both alloys studied with respect to both temperature and solute content. As supported by the literature, boundaries exhibit a maximum mobility for a 38-39°<111> misorientation in initial annealing experiments; this mobility maximum is asymmetric with a sharp cutoff below 38-39° but a more gradual decrease at misorientations beyond 40°. The presence of a minimum at 38-39° is found at both higher temperatures and higher solute concentrations. A shift in texture dependency with solute and temperature is also observed. This transition from a local mobility maximum to a minimum is discussed within the context of recent developments in solute drag theory.
Optical characterization of hierarchically self-assembled GaAs/AlGaAs quantum dots
Authors: Marabelli F. , Rastelli A. , Schmidt O. G. , Beaurin G. , Geddo M. , Guizzetti G.
Published: April 2011

Recently, a simple method has been developed to obtain inverted GaAs/AlGaAs quantum dots (QDs) below a quantum well (QW) via multi-step (hierarchical) self-assembly [1]. Here we report on the optical characterization of a series of structures GaAs QDs with different size. The study is performed by means of reflectance (R) down to T=15 K, photo- and thermo-reflectance (T>80 K) and by spectroscopic ellipsometry (at RT). Such measurements allow us to check the thickness and composition of the barrier layers and to complete the study of the electronic states involved in the emission properties of QDs and QW levels. QW states were well identified and their energies and shift with the size parameters well agree with the photoluminescence (PL) data. The identification of QD states seems less straightforward in R measurements and deserves further investigation.
Nitrogen induced optical phonon shift in GaNyAs1-y studied by Raman Scattering
Authors: Tay Li-Lin , Lockwood David J. , Gupta James A.
Published: April 2011

We present a Raman study of pseudomorphically strained epitaxial films of the ternary alloy GaNyAs1-y grown on GaAs(100) with y ranging from 0 to 0.06. The optical phonon Raman spectrum of the alloy displays a two-mode behavior. The GaAs-like first order modes for y = 0.06 are represented by the strong longitudinal optic (LO1) mode at 287.4 cm-1 and the weaker transverse optic (TO1) mode at 269.0 cm-1, while the GaN-like LO2 mode is observed at 475.6 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 in the alloy increases, the GaAs-like LO1 band shifts linearly towards lower wavenumber while the linearly increasing GaN-like LO2 phonon band deviates from linearity at higher nitrogen concentration (y ≥ 0.03). The reason for the deviation of the LO2 phonon from linearity is discussed.
Improved performance of GaSb-based MIR photodetectors through electrochemical passivation in sulphur containing solutions
Authors: Piotrowska A. , Papis E. , Golaszewska K. , Lukasiewicz R. , Kaminska E. , Piotrowski T. T. , Kruszka R. , Kudla A. , Rutkowski J. , Szade J. , Winiarski A. , Wawro A. , Aleszkiewicz M.
Published: April 2011

Sulfur passivation effects on the performance of LPE-grown GaSb/InGaAsSb/AlGaAsSb mesatype photodiodes operating in wavelengths range 1.9 – 2.3 μm have been investigated. (NH4)2S, Na2S, and (NH2)2CS have been chosen as sulfur sources in either aqueous or C3H7OH solutions. Electrochemical passivation of mesa side walls was proven to reduce photodiodes dark current and increasing their differential resistance by a factor of 4. As a result devices characterized by the detectivity of 1.5–2×1010 cmHz1/2/W and dark current density of 20 mA/cm2 at –0.5V bias have been fabricated and their long-term stability has been proven.
High Speed Ge Photodetectors on Si Platform for GHz Optical Communications in C+L Bands
Authors: Liu Jifeng , Michel Jurgen , Cannon Douglas D. , Giziewicz Wojciech , Pan D. , Danielson David T. , Jongthammanurak Samerkhae , Yasaitis John , Wada Kazumi , Fonstad Clifton G. , Kimerling Lionel C.
Published: April 2011

We present a high speed, high responsivity, tensile strained Ge p-i-n photodetector selectively grown on Si platform that covers the whole C band and a large part of the L band for high capacity optical communications. The device shows a 3dB bandwidth of 2.5GHz and its responsivities at 1310nm and 1550nm are comparable to commercial InGaAs photodetectors currently used in telecommunications. The device has promising applications in Si microphotonics such as the fiber-to-the-home technology.
Towards p-type doping of ZnO by ion implantation
Authors: Coleman V. A. , Tan H. H. , Jagadish C. , Kucheyev S. O. , Phillips M. R. , Zou J.
Published: April 2011

Zinc oxide is a very attractive material for a range of optoelectronic devices including blue light-emitting diodes and laser diodes. Though n-type doping has been successfully achieved, p-type doing of ZnO is still a challenge that must be overcome before p-n junction devices can be realized. Ion implantation is widely used in the microelectronics industry for selective area doping and device isolation. Understanding damage accumulation and recrystallization processes is important for achieving selective area doping. In this study, As (potential p-type dopant) ion implantation and annealing studies were carried out. ZnO samples were implanted with high dose (1.4 × 1017 ions/cm2) 300 keV As ions at room temperature. Furnace annealing of samples in the range of 900°C to 1200°C was employed to achieve recrystallization of amorphous layers and electrical activation of the dopant. Rutherford backscattering/channeling spectrometry, transmission electron microscopy and cathodolumiescence spectroscopy were used to monitor damage accumulation and annihilation behavior in ZnO. Results of this study have significant implications for p-type doing of ZnO by ion implantation.
Latest developments in Blue-Violet Laser Diodes grown by Molecular Beam Epitaxy
Authors: Bousquet V. , Kauer M. , Johnson K. , Zellweger C. , Hooper S.E. , Heffernan J.
Published: April 2011

We report on recent results obtained for InGaN multiple quantum well laser diodes grown by ammonia based Molecular Beam Epitaxy. The laser diodes were grown on freestanding GaN substrates and operated at room temperature under pulsed current injection conditions. For devices with improved p-type doping a threshold current density of 6.7kA.cm−2 was measured for a current pulse duration of 200ns and an operating temperature of 3.9°C. A duty cycle up to 50% with a pulsed injection current duration of 500μs was also achieved at 3.9°C.
Advances in AlGaN-based Deep UV LEDs
Authors: Crawford M. H. , Allerman A. A. , Fischer A. J. , Bogart K. H. A. , Lee S. R. , Chow W. W. , Wieczorek S. , Kaplar R. J. , Kurtz S. R.
Published: April 2011

Materials studies of high Al-content (> 30%) AlGaN epilayers and the performance of AlGaN-based LEDs with emission wavelengths shorter than 300 nm are reported. N-type AlGaN films with Al compositions greater than 30% reveal a reduction in conductivity with increasing Al composition. The reduction of threading dislocation density from the 1–5 × 1010 cm-2 range to the 6–9 × 109cm-2 range results in an improvement of electrical conductivity and Al0.90Ga0.10N films with n= 1.6e17 cm-3 and μ=20 cm2/Vs have been achieved. The design, fabrication and packaging of flip-chip bonded deep UV LEDs is described. Large area (1 mm × 1 mm) LED structures with interdigitated contacts demonstrate output powers of 2.25 mW at 297 nm and 1.3 mW at 276 nm when operated under DC current. 300 μm × 300 μm LEDs emitting at 295 nm and operated at 20 mA DC have demonstrated less than 50% drop in output power after more than 2400 hours of operation. Optimization of the electron block layer in 274 nm LED structures has enabled a significant reduction in deep level emission bands, and a peak quantum well to deep level ratio of 700:1 has been achieved for 300 μm × 300 μm LEDs operated at 100 mA DC. Shorter wavelength LED designs are described, and LEDs emitting at 260 nm, 254nm and 237 nm are reported.
Efficient and Reliable Homoepitaxially-Grown InGaN-Based Light-Emitting Diodes
Authors: Cao X. A. , Teetsov J. M. , LeBoeuf S. F. , Arthur S. D. , Kretchmer J.
Published: April 2011

InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) with peak emission at ∼405 nm were grown on bulk GaN and sapphire substrates using MOCVD. Tunneling current was found to be dominant in the LED on sapphire over a wide range of applied bias, but was substantially suppressed in the homoepitaxial LED. Nanoscale electrical characterization using conductive atomic force microscopy (C-AFM) revealed highly localized leakage current at V-defects in the LED structure on sapphire, indicating that the associated threading dislocations were electrically active and behaved as leakage current pathways connected across the p-n junction. Compared to the lateral LED on sapphire, the vertically-structured LED on GaN had a reduced series resistance of 7 Ω and greatly improved power conversion efficiencies. The homoepitaxial LED was subjected to stress test at 400 mA for 24 h and showed minimal optical degradation, whereas the same stress resulted in the destruction of the LED on sapphire due to increased current crowding and self-heating.
A Model for the Critical Height for Dislocation Annihilation and Recombination in GaN Columns Deposited by Patterned Growth
Authors: Twigg M. E. , Bassim N. D. , Eddy C. R. , Henry R. L. , Holm R. T. , Mastro M. A.
Published: April 2011

In order to reduce vertical leakage in III-nitride detectors, we have grown a patterned array of hexagonal GaN columns on masked heteroepitaxial GaN template layers using a-plane sapphire substrates. In addition to eliminating cracking, we have found that for GaN columns tens of microns in diameter and several microns high, the dislocation density is also significantly reduced. We have developed a simple closed-form analytical model for predicting the critical column height for the onset of the reduction in the dislocation density. Among the predictions of this model is that the critical column height for the onset of dislocation density reduction is proportional to the product of column width and the grain size of the GaN film.
Growth of GaN from elemental Gallium and Ammonia via Modified Sandwich Growth Technique
Authors: Berkman E. , Collazo R. , Schlesser R. , Sitar Z.
Published: April 2011

Gallium nitride (GaN) films were grown on (0001) sapphire substrates at 1050°C by controlled evaporation of gallium (Ga) metal and reaction with ammonia (NH3) at a total reactor pressure of 800 Torr. Pure nitrogen (N2) was flowed directly above the molten Ga source to prevented direct reaction between the molten Ga and ammonia, which causes Ga spattering and GaN crust formation. At the same time, this substantially enhanced the Ga transport to the substrate. A simple mass-transport model based on total reactor pressure, gas flow rates and source temperature was developed and verified. The theoretical calculations and growth rate measurements at different ammonia flow rates and reactor pressures showed that the maximum growth rate was controlled by transport of both Ga species and reactive ammonia to the substrate surface.
Growth of Uncracked Al0.80Ga0.20N/GaN DBR on Si(111)
Authors: Charles M. B. , Kappers M. J. , Humphreys C. J.
Published: April 2011

The strain in Al0.80Ga0.20N/GaN distributed Bragg reflectors (DBRs) grown by metalorganic vapour phase epitaxy (MOVPE) on Si(111) was studied using high resolution X-ray diffraction (XRD). Previous studies have shown that a 50nm Al0.80Ga0.20N layer induced a compressive strain in Al0.12Ga0.88N capping layers and prevented crack formation. A seven period Al0.80Ga0.20N/GaN DBR was grown, but this was found to be cracked at room temperature, despite compression in the GaN layers. This problem was solved by growing an identical structure with the addition of a 650nm GaN cap, and due to the compression in this layer, the structure was crack-free.
Polarity Control of GaN Films Grown by Metal Organic Chemical Vapor Deposition on (0001) Sapphire Substrates
Authors: Mita Seiji , Collazo Ramon , Schlesser Raoul , Sitar Zlatko
Published: April 2011

The polarity control of GaN films grown on c-plane sapphire substrates by low pressure metal organic chemical vapor deposition (MOCVD) was achieved by using N2 as a diluent and transport gas. The type of polarity was governed by the substrate treatment prior to the GaN growth. N-face (-c) GaN films were only obtained by pre-nitridation of the sapphire substrate after a H2 anneal, while Ga-face (+c) GaN films were grown directly on the substrates or on properly annealed AlN buffer layers. In addition, GaN films on improperly annealed AlN buffer layers, that is, under- or over-annealed buffer layers, yielded films with mixed polarity. Smooth N-face GaN films with 2.5 nm RMS roughness, as determined by atomic force microscopy (AFM), were obtained with shorter nitridation times (less than 2 min). Wet chemical etching in an aqueous solution of potassium hydroxide (KOH) was used to determine the polarity type.
Near Field Optical Spectroscopy of GaN/AlN Quantum Dots
Authors: Neogi A. , Gorman B. P. , Morkoç H. , Kawazoe T. , Ohtsu M. , Kuball M.
Published: April 2011

We investigate the spatial distribution and emission properties of self-assembled GaN/AlN quantum dots. High-resolution transmission electron microscopy reveals near vertical correlation among the GaN dots due to a sufficiently thin AlN spacer layer thickness, which allows strain induced stacking. Scanning electron and atomic force microscopy show lateral coupling due to a surface roughness of ∼ 50–60 nm. Near-field photoluminescence in the illumination mode (both spatially and spectrally resolved) at 10 K revealed emission from individual dots, which exhibits size distribution of GaN dots from localized sites in the stacked nanostructure. Strong spatial localization of the excitons is observed in GaN quantum dots formed at the tip of self-assembled hexagonal pyramid shapes with six [101 1] facets.
Resonantly enhanced second harmonic generation in a one-dimensional GaN-based photonic crystal slab
Authors: Torres Jérémi , d'Yerville Marine Le Vassor , Cassagne David , Legros René , Lascaray Jean-Paul , Centeno Emmanuel , Albert Jean-Paul , Coquillat Dominique
Published: April 2011

We have performed investigations of resonance effects inside a gallium nitride one-dimensional photonic crystal slab in order to enhance the second-harmonic generated from an beam incident on the surface of the slab. Convenient conditions on the incident beam propagation direction and polarization are first identified by experimental or theoretical linear optical studies. Giant enhancements in the second-harmonic conversion have been obtained by comparison with the unpatterned GaN layer. The combined role of the resonant coupling of the fundamental field and of the second-harmonic field has been observed by rotating the polarization of the fundamental beam.
Microstructure of highly p-type doped GaN sub-contact layers for low-resistivity contacts
Authors: Kröger R. , Dennemarck J. , Böttcher T. , Figge S. , Hommel D.
Published: April 2011

The effect of a highly Mg doped subcontact layer on top of GaN grown by metal organic vapor phase epitaxy and its interface to a Pd/Au contact layer was investigated by means of transmission electron microscopy and electrical characterization techniques. Use was made of prior investigations of the Mg doping characteristics, which showed the existence of a segregation related defect free layer even for doping levels as high as 5×10−19cm−3, which thickness depends on the Mg to Ga molar precursor flow ratio. For a given subcontact layer thickness of 15 nm a critical precursor molar flow ratio of 0.035 resulted in a smooth surface showing an interfacial layer indicating a Pd/Ga alloying. This layer resulted in a contact resistivity as low as 2×10−5 Ωcm2. For a flow ratio of 0.070 the surface was found to be rough due to defect formation resulting in a contact resistivity as high as 10−3 Ωcm2 similar to the value obtained without subcontact layer. Moreover, the metallization layer showed in all cases a texture of the {111} lattice planes with respect to the {0002} planes of the GaN.
Impact of H2-preannealing of the Sapphire Substrate on the Crystallization of Low-Temperature-Deposited AlN Buffer Layer
Authors: Tsuda Michinobu , Balakrishnan Krishnan , Iwaya Motoaki , Kamiyama Satoshi , Amano Hiroshi , Akasaki Isamu
Published: April 2011

The effect of H2-preannealing of sapphire substrate on low-temperature (LT- ) AlN buffer layer deposited by metalorganic vapor phase epitaxy is investigated. Crystallinity of LT-AlN drastically changes with preannealing temperature variation. It is found that H2-preannealing of sapphire substrate is a requisite to get a better quality LT-AlN and as a consequence it leads to growth of better quality GaN epi-layer on it.
Evidence of Carrier Mediated Ferromagnetism in GaN:Mn/GaN:Mg Heterostructures
Authors: Erdem Arkun F. , Reed Mason J. , Berkman Erkan Acar , El-Masry Nadia A. , Zavada John M. , Oliver Luen M. , Reed Meredith L. , Bedair Salah M.
Published: April 2011

Dilute Magnetic Semiconductors (DMS's) posses a strong potential to make use of the spin of carriers in spintronic devices. Experimental results and theoretical calculations predict that GaN:Mn is a potential semiconductor material for spintronic device applications. The dependence of the room temperature ferromagnetic properties of GaN:Mn/GaN:Mg double heterostructures (DHS) on the Fermi level position in the crystal is demonstrated. Several GaN:Mn/GaN:Mg DHS are grown by metal organic chemical vapor deposition on sapphire. It is shown that initially paramagnetic films can be rendered ferromagnetic by facilitating carrier transfer through the GaN:Mn/GaN:Mg interface. Additionally, it is demonstrated that ferromagnetism depends on the thickness of the GaN:Mn and GaN:Mg layers. The carrier transfer process essentially changes the Fermi level position in the crystal. By choosing the right thicknesses for GaN:Mn and GaN:Mg an optimum DHS that exhibits room temperature ferromagnetism is grown. An identical structure, with the exception of insertion of an AlGaN barrier in order to obstruct the carrier transfer at the interface, results in paramagnetic films for AlGaN barriers thicker than 25nm. These results are explained based on the change in the occupancy of the 3d-Mn impurity band, and indicate that carrier mediation is the possible mechanism for the ferromagnetism observed in the MOCVD grown GaN:Mn material system. This is the first evidence that this material system responds to electronic perturbations, hence ferromagnetism observed is not due to secondary phases or spin glass behavior.
Raman Gain in Silicon Using Highly Confined Waveguide Structure
Authors: Xu Qianfan , Almeida Vilson R. , Lipson Michal
Published: April 2011

We show ultra-fast Raman gain dynamics in submicron-size silicon-on-insulator strip waveguide. Using high confinement structures and pico-second pump pulses, we show 6-dB net nonlinear gain with 20.7-W peak pump power in a 7-mm long waveguide.
Classical versus ab initio structural relaxation: electronic excitations and optical properties of Ge nanocrystals embedded in a SiC matrix
Authors: Cappellini Giancarlo , Weissker H.-Ch. , De Salvator D. , Furthmüller J. , Bechstedt F. , Satta G. , Casula F. , Colombo L.
Published: April 2011

We discuss and test a combined method to efficiently perform ground- and excited-state calculations for relaxed structures using both a quantum first-principles approach and a classical molecular-dynamics scheme. We apply this method to calculate the ground state, the optical properties, and the electronic excitations of Ge nanoparticles embedded in a cubic SiC matrix. Classical molecular dynamics is used to relax the large-supercell system. First-principles quantum techniques are then used to calculate the electronic structure and, in turn, the electronic excitation and optical properties. The proposed procedure is tested with data resulting from a full first-principles scheme. The agreement is quantitatively discussed between the results after the two computational paths with respect to the structure, the optical properties, and the electronic excitations. The combined method is shown to be applicable to embedded nanocrystals in large simulation cells for which the first-principle treatment of the ionic relaxation is presently out of reach, whereas the electronic, optical and excitation properties can already be obtained ab initio. The errors incurred from the relaxed structure are found to be non-negligible but controllable.
Theory of large-scale electronic structure calculation and nanostructures formed in silicon cleavage simulation: surface reconstruction, step and bending
Authors: Hoshi Takeo , Iguchi Yusuke , Fujiwara Takeo
Published: April 2011

Several methodologies are developed for large-scale atomistic simulations with fully quantum mechanical description of electron systems. The important methodological concepts are (i) generalized Wannier state and (ii) Krylov subspace. Test calculations are done with upto 106 atoms using a standard workstation. As a practical nanoscale calculation, the 10 nm scale structure in cleavage process of silicon is investigated. Discussions are focused mainly on the stable (experimentally observed) (111)-(2 × 1) cleavage process. Surface reconstruction and step formation are observed and compared with experiments. These processes are analyzed by the quantum mechanical freedoms of electron system, such as the local density of states (LDOS). We also observed the bending of cleavage plane into the (111) plane from other planes, which is a direct evidence of the stability of the (111) cleavage mode. Finally, several common aspects between cleavage and other phenomena are discussed from a general viewpoint of 10 nm scale structure.
Diamondlike Carbon-Metal Nanocomposite Films for Medical Applications
Authors: Narayan R. J. , Abernathy H. , Riester L. , Berry C. J. , Brigmon R.
Published: April 2011

Silver and platinum were incorporated within diamondlike carbon (DLC) thin films using a multicomponent target pulsed laser deposition process. Transmission electron microscopy of the DLC-silver and DLC-platinum composite films reveals that these films self-assemble into particulate nanocomposite structures that possess a high fraction of sp3-hybridized carbon atoms. DLC-silver-platinum films demonstrated exceptional antimicrobial properties againstStaphylococcusandPseudomonas aeruginosabacteria.
Comparison of -OH and -NH2 Functional Group Substitution on PTFE Surface with V-UV Photon Irradiation for Protein Adsorption
Authors: Sato Yuji , Kobayashi Naoki , Murahara Masataka
Published: April 2011

Poly-tetrafluoroethylene [PTFE] presents few rejections in a living body but has low tissue affinity. Then, the soft tissue implant material that has not only high biocompatibility but also superb bondability has been developed by photo-chemically substituting the hydrophilic of –OH or –NH2 groups on the PTFE surface with V-UV photon irradiation. The protein adsorption of the sample before and after treatment was also evaluated by scanning electron microscope [SEM] and attenuated total reflection Fourier-transform infrared [ATR FT-IR], using bovine serum albumin [ALB] and fibrin [FIB] solution as a protein index in biocompatibility test. From the results, it has been confirmed that the protein adsorption increased with the increase in the hydrophilic group's substitution density. The -OH incorporated sample adsorbed the ALB and FIB more than the -NH2 incorporated sample; the amount of the ALB and FIB sticking became 2.3 times larger than that of the non-treatment sample.
High-speed Processing with Reactive Cluster Ion Beams
Authors: Seki Toshio , Matsuo Jiro
Published: April 2011

Cluster ion beam processes can produce high rate sputtering with low damage in comparison with monomer ion beam processes. Especially, it is expected that extreme high rate sputtering can be obtained using reactive cluster ion beams. Reactive cluster ion beams, such as SF6, CF4, CHF3, and CH2F2, were generated and their cluster size distributions were measured using Time-of-Flight (TOF) method. Si substrates were irradiated with the reactive cluster ions at the acceleration energy of 5–65 keV. Each sputtering yield was increased with acceleration energy and was about 1000 times higher than that of Ar monomer ions. The sputtering yield of SF6 cluster ions was about 4600 atoms/ion at 65 keV. With this beam, 12 inches wafers can be etched 0.5 μm per minute at 1 mA of beam current. The TOF measurement showed that the size of SF6 cluster was about 550 molecules and the number of fluorine atoms in a SF6 cluster was about 3300. If the sputtered product was SiF, the yield has to be less than 3300 atoms/ion. These results indicate that the reactive cluster ions etch targets not only chemically, but also physically. This high-speed processing with reactive cluster ion beam can be applied to fabricate nano-devices.
Characterization of nanocrystalline surface layer induced by shot peening and effect on their fatigue strength.
Authors: Mano Hideo , Satoru Kondo , Matsumuro Akihito , Imura Toru
Published: April 2011

The shot peening process is known to produce a hard layer, known as the white layer” on the surface of coil springs. However, little is known about the fatigue properties of this white-layer.

In this study, coil springs with a white-layer were manufactured. The surface of these springs was then examined using micro Vickers hardness, FE-SEM etc. to test fatigue strength of the springs.

From the results obtained, a microstructure of the white-layer with grain size of 50–100 nm was observed, with a Vickers hardness rating of 8–10 GPa.

Tow category springs were manufactured utilizing a double-peening process. These springs had the same residual stress destruction and surface roughness. Only one difference was observed: one spring had a nanocrystalline layer on the surface, while the other did not. The results of the fatigue test realized an increase in the fatigue life of the nanocrystalline surface layer by 9%.