bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of MeV Ions on Thermal Stability of Single Walled Carbon Nanotubes
Authors: Adhikari Ananta Raj , Huang Mengbing , Ryu Chang , Ajayan Pullickel , Bakhru Hassaram
Published: April 2011

The properties of carbon nanotubes (CNTs) are closely dependent on their structures, and therefore may be tailored by controllably introducing defects in the nanotube systems. In this work, we have investigated the effects of energetic ions (H+ and He+) on the thermal stability of single wall nanotubes (SWNTs) against oxidation in air. SWNTs were irradiated with MeV ions to various doses in the range 1013-1016 cm−2. Thermogravimetric analysis (TGA) was used to determine the loss of CNT masses as a result of oxidation processes. As opposed to the case of pristine SWNTs for which the temperature (Tmax) corresponding to maximum oxidation rate was found to be ∼ 495 °C, ion beam processing significantly enhanced the thermal stability of nanotubes, e.g., Tmax increased by about 30 °C after H+ implantation (dosage: 1015 cm−2) and 17 °C after He+ implantation (dosage: 1013 cm−2). The activation energies for thermal oxidation under various conditions were also extracted from TGA data, with values ranging from 1.13 eV (for pristine SWNTs) to 1.37 eV, depending on ion doses and species. Raman spectroscopy was used to determine the characteristics of the G band (C-C stretching mode) and D band (disorder induced mode) in CNTs. The work suggests that the SWNTs modifies to more stable structure (may be cross-linked SWNTs) at small doses. Once the number of defects exceeds some critical value (depending on the type and dosage of bombarding ion) the bonding energy in CNTs weakens, leading to the reduced thermal stability of CNTs against oxidation.
Magnetic Aging
Authors: Skomski R. , Zhou J. , Kirby R. D. , Sellmyer D. J.
Published: April 2011

Thermally activated magnetization reversal is of great importance in areas such as permanent magnetism and magnetic recording. In spite of many decades of scientific research, the phenomenon of slow magnetization dynamics has remained partially controversial. It is now well-established that the main mechanism is thermally activated magnetization reversal, as contrasted to eddy currents and structural aging, but the identification of the involved energy barriers remains a challenge for many systems. Thermally activated slow magnetization processes proceed over energy barriers whose structure is determined by the micromagnetic free energy. This restricts the range of physically meaningful energy barriers. An analysis of the underlying micromagnetic free energy yields power-law dependences with exponents of 3/2 or 2 for physically reasonable models, in contrast to arbitrary exponentsmand to 1/H-type laws.
Degradation in Sn Films due to Whisker Formation
Authors: Reinbold L. , Chason Eric , Jadhav N. , Kelly V. , Holmes P. , Shin J.W. , Chan W.L. , Kumar K.S. , Barr G.
Published: April 2011

Whisker formation in pure Sn coatings on Cu conductors is a serious impediment to the development of Pb-free electronics manufacturing. Understanding whisker formation is complicated by the fact that it is the result of multiple materials kinetic processes including interdiffusion, intermetallic formation and stress generation We report preliminary studies of whisker growth kinetics and stress evolution aimed at developing a fundamental understanding of the whisker growth process. A proposed model of point defect mediated stress generation provides a simple picture of how the different processes are connected.
A new method based on work-displacement curve to assess the toughness of coated system
Authors: Chen Jinju , Bull Steve J.
Published: April 2011

Traditional indentation models relating the toughness to applied load and the length of well-developed radial cracks were initially developed for monolithic bulk brittle materials. Their application can also be extended to coated systems if sufficiently thick, however, the use of such models to thin coatings will be influenced by other factors such as substrate deformation and it is difficult to use traditional approaches to assess sub-threshold cracking. The method of extrapolating the load-displacement curve has been widely used to assess the toughness of a coating when a step in the load-displacement curve is observed which is clearly associated with chipping. This approach completely ignores the change of other mechanical behaviour associated with the fracture event and it cannot be applied to displacement control experiments. The plot of the irreversible work vs. load for a range of applied loads can be used in this case but is time consuming. Also this method does not work well for sub-threshold cracking. Therefore, it is necessary to find a method to solve such problems if the fracture properties of very thin coatings (≤1μm) are to be assessed. In this paper, a method which is based on extrapolating the total work vs. displacement curve at a load where cracking occurs is presented to estimate the fracture toughness of the coated system.
The Influence Of Substrate On Partially Rapidly Solidified Alumina-3 wt % titania Nanocrystalline Coatings Deposited by Plasma Spray Technique
Authors: Venkataraman R. , Parmanick B. , Ravikumar B. , Pathak L. C. , Das D. K. , Venkataraman B
Published: April 2011

In this paper an attempt was made to impose different degrees of rapid solidification by spraying on diverse substrates of varying thermal properties. Substrates such as Copper, Aluminum, Stainless steel, Low alloy steel substrates were used to alter the imposed cooling rate and thereby the amount of residual α phase. A start powder of 3 wt % Alumina-titania powder was used for spraying to a thickness of 250 μm on the different substrates specified. In all cases the rapidly solidified phases show nanocrystalline sizes with the most rapidly solidified metastable γ phase showing finer grain size of less than 25 nm. The surface roughness of the substrate and the coating were characterized by Atomic force microscopy. In contrary to the Alumina-13 wt % titania, coupons of Alumina-3 wt % titania had shown poor indentation fracture toughness with increased amount of residual α phase. Coupons of stainless steel and low alloy steel had shown the lowest fracture toughness when tested by Vickers type indentation at loads of 3 N and 5 N. In contrast to these results the interfacial toughness when measured by Rockwell indentation technique at loads of 150 N was found to be dependent on the elastic modulus of the substrate more than the coating hardness. The interfacial toughness was found to be lower for softer material such as aluminum and copper than stainless steel and low alloy steel.
In Situ Optical Tribometry Studies of Nanocomposite Coatings
Authors: Chromik Richard R. , Baker Colin C. , Voevodin Andrey A. , Wahl Kathryn J.
Published: April 2011

Six nanocomposite coatings of yttria-stabilized zirconia (YSZ), Au, diamond like carbon and MoS2 have been studied by in situ tribometry. The coatings all had a nominal MoS2 content of between 14 and 18 mol%, while the concentrations of the other components were varied. Steady state friction coefficients in both dry (0.04–0.05) and wet sliding conditions (0.06–0.08) were similar for all coatings. However, by in situ tribometry, wear mechanisms were found to differ depending on coating composition and test environment humidity. Friction spiking in high YSZ coatings was identified with local plowing and scoring of the track followed by an extrusion of transfer film material. Trends in coating hardness and modulus are correlated to composition and coating wear processes.
Reactive ion etching of TiN, TiAlN, CrN and TiCN Films in CF4/O2 and CHF3/O2 Plasmas
Authors: Leech Patrick W. , Reeves G. K. , Holland A. S.
Published: April 2011

The reactive ion etching of a range of hard coatings (TiN, TiCN, CrN and TiAlN) has been examined as a function of rf power, flow rate and pressure. The films were deposited by filtered arc deposition (TiN, TiAlN and CrN) or low energy electron beam (TiCN) on polished disc substrates of M2 tool steel. The flat surfaces were lithographically patterned with a grating structure (∼1 μm pitch). The TiN and TiCN layers have shown significantly higher etch rates (100-250 nm/min) than the CrN and TiAlN (∼5 nm/min) coatings. These regimes of higher and low etch rate were identified as ion-enhanced chemical etching and physical sputtering, respectively. In CF4/O2 plasma, the etch rate of the TiN and TiCN layers increased with rf power, flow rate and pressure which were parameters known to enhance the density of active fluorine species. The etch rates of TiN and TiCN layers were higher in CF4/O2 plasma than in CHF3/O2 gases in which polymer deposition was produced at pressure ≥ 35 mTorr.
Effects of Preferred Orientation on the Tribological Behavior of the TiN and CrN Films by the CAIP
Authors: Bae Sang Geun , Cho Yong Ki , Moon Kyoung Il , Kim Sang Gweon , Kim Sung Wan
Published: April 2011

For the better understanding of anti-wear properties of hard coating films, the effects of preferred orientation, surface roughness, and surface hardness on the tribological property of TiN and CrN films have been studied. TiN and CrN films were deposited by cathodic AIP (CAIP) with working temperatures of 350∼450°C, working pressures of 1∼5Pa, arc currents of 50∼90 A, and bias voltages of 30∼150V. The characteristics of microstructure and its effect on the friction coefficient were investigated with working conditions. The preferred orientation of the film was examined by XRD. The friction coefficient was determined using ball on disk tribometer. The preferred orientation of films was changed from (200) to (111) with decreasing working pressure and increasing bias voltage. It is reported that TiN coating with strong (111) orientation have better tribological property, but, in this study, although similar tendency was also found in TiN, the tribological property was increased with diminishing (111) orientation in CrN films. It is concluded in this study that the tribological property is not affected only by the orientation of the film, but it depends on the combined properties of surface roughness, surface hardness, and orientation of the film. The friction coefficients of TiN and CrN were 0.48∼0.52 and 0.49∼0.53, respectively.
Selective protein adsorption on highly hydrophilic surface modified with combined V–UV photon and plasma irradiation
Authors: Sato Yuji , Murahara Masataka
Published: April 2011

A highly hydrophilic surface, with a water contact angle smaller than 20 degrees, was rendered to a poly-methylmethacrylate [PMMA] by Xe2 excimer lamplight and plasma treatment. In result, the implant material that was highly biocompatible and adhesive to soft tissue was developed. The PMMA surface, which had been pretreated by ammonia plasma, was irradiated with Xe2 excimer lamplight in the presence of hydrazine as a reaction solution. In order to evaluate the wettability of the modified sample, the contact angle with water was measured. The contact angle was decreased from 83 degrees for untreated PMMA to 60 degrees when irradiated the Xe2 lamplight for 30 minutes. With the ammonia plasma pretreatment, the contact angle of the treated sample became to 7 degrees at the 1 minute Xe2 lamp irradiation.
Stress induced phase transition in a monomolecular perfluroalkylsilane film self assembled on aluminium surface
Authors: Biswas Sanjay Kumar , Devaprakasam D.
Published: April 2011

We control the stiffnesses of two dual double cantelevers placed in series to control penetration into a perflurooctyltrichlorosilane monolayer self assembled on aluminium and silicon substrates. The top cantilever which carries the probe is displaced with respect to the bottom cantilever which carries the substrate, the difference in displacement recorded using capacitors gives penetration. We further modulate the input displacement sinusoidally to deconvolute the viscoelastic properties of the monolayer. When the intervention is limited to the terminal end of the molecule there is a strong viscous response in consonance with the ability of the molecule to dissipate energy by the generation of gauche defects freely. When the intervention reaches the backbone, at a contact mean pressure of 0.2GPa the damping disappears abruptly and the molecule registers a steep rise in elastic modulus and relaxation time constant, with increasing contact pressure. We offer a physical explanation of the process and describe this change as due to a phase transition from a liquid like to a solid like state.
Tribological Properties of Sputter-Deposited MoS2 Films Containing Titanium
Authors: Krzanowski James , Nunna Dyumani
Published: April 2011

The tribological properties of sputter-deposited MoS2 and MoS2-Ti films were investigated in this study. The deposited films were characterized using microprobe analysis for composition and x-ray diffraction (XRD) for structure. The frictional properties of the films were examined using a pin-on-disk (POD) with counterfaces of 440C steel, aluminum, tungsten carbide and alumina. The tests were run under low (25%), medium (50%) and high (70%) humidity levels. MoS2 films without Ti were first examined under cyclic humidity conditions between 25 and 50% R/H. The results showed that for steel, WC and alumina counterfaces, the effect of the higher humidity was to increase the friction, but lower friction could be recovered when the humidity was reduced back to 25%. For films containing Ti, the best results were obtained at a concentration of 20 at. % Ti. These films performed well for steel and WC counterfaces, but poorly against aluminum. The effect of deposition temperature (up to 450oC) was examined for MoS2 and MoS2-5% Ti films. Higher temperatures yielded more crystalline films, but the addition of Ti partially countered this effect. The POD test showed that at medium humidity levels the friction decreased with temperature, but increased slightly when tested under low humidity. In all cases, the 5% Ti-containing films had a fiction coefficient of about 0.1 below that for films without Ti.
IV-VI Semiconductor Mid-IR Lasers
Authors: McCann Patrick J. , Selivanov Yurii
Published: April 2011

Mid-IR lasers fabricated from narrow bandgap IV-VI semiconductors are proven devices for molecular spectroscopy applications. Wide single-mode tunability, low waste heat generation, and large spectral coverage from about 1000 cm−1 (10 µm) to about 2500 cm−1 (4 µm) have allowed development of laser absorption spectroscopy instrumentation for fast and sensitive measurement of specific gas phase molecules. For example, IV-VI mid-IR lasers with emission in the 5.2 µm spectral range have recently enabled the development of breath analysis instruments for real-time measurement of exhaled nitric oxide (eNO). Laser tunability with current ramping is sufficient for simultaneous measurement of exhaled carbon dioxide (eCO2), a capability that allows highly accurate determination of eNO concentrations in the low ppb range, a sensitivity required for assessing airway inflammation in patients with asthma. After discussing emerging medical diagnostic applications this paper reviews recent progress in the development of liquid-nitrogen-free cryogenic cooling systems for IV-VI mid-IR lasers. A description of continuing research on the development of improved IV-VI lasers, where the primary objective is to fabricate devices with continuous wave (cw) operation at room temperature is then presented. Theoretical and experimental analysis of (111)-oriented multiple quantum well (MQW) IV-VI materials show that it should be possible to reduce lasing thresholds significantly. In addition, results from transferring IV-VI materials from low thermal conductivity growth substrates to higher thermal conductivity copper show that new laser packaging methods can significantly improve active region heat dissipation. Together, these new materials and device packaging methods promise to enable the fabrication of IV-VI mid-IR lasers with cw operation at room temperature.
Wavefunction Engineering of Layered Quantum Semiconductor Structures: Recent Progress
Authors: Ram-Mohan L. R.
Published: April 2011

A Lagrangian formulation for the valence bands of bulk zinc blende and wurtzite compound semiconductors provides a direct approach to determining derivative operator ordering at layer interfaces in the multibandk·Pdescription of electronic states, in the envelope function approximation. The current continuity condition is obtained through a gauge-variation on the Lagrangian. The principle of least action together with the discretization of the action integral naturally leads into a finite element approach for the modeling and simulation of the multiband Schrödinger equations. Being a variational method, the wavefunctions and energy eigenvalues can be systematically improved with quadratic convergence. By including the Poisson Lagrangian, a self-consistent treatment of the Schrödinger-Poisson band-bending in arbitrarily doped structures is obtained. Numerical examples are presented for the valence band electronic states for quantum wells and superlattices. We have also coupled the finite element method with layer parameter optimizations to design heterostructures for lasers emitting at given wavelengths. It is in this sense that wavefunction engineering goes beyond the notion of bandgap engineering in providing the means for optimizing wavefunction localization in layers, selecting layer thicknesses, enhancing optical matrix elements, and laser gain.
Spatial Bandgap Tuning in Long Wavelength InAs Quantum Dots-in-Well Laser Structure
Authors: Wang Yang , Dimas Clara E. , Djie Hery S. , Ooi Boon S. , Dang Gerard , Chang Wayne
Published: April 2011

We employed the postgrowth impurity-free vacancy diffusion technique to selectively tune the bandgap of the InAs/InGaAlAs dots-in-well laser structure grown on (100) InP substrate. A blueshift up to 170 nm with a significant decrease in the photoluminescence linewidth has been observed. Spatial control of the bandgap shifts has been achieved using SiO2 and SixNy layers as annealing caps. A differential wavelength shift of 76 nm has been observed after a rapid thermal annealing step at 750 °C for 30 s. In contrast to most reported results in other material systems using similar process, we observed a larger bandgap shift from the SixNy capped samples than from the SiO2 capped samples. Our theoretical calculation indicates that the unusual intermixing behavior in this material system is governed by different interdiffusion rates of group-III atoms.
Annealing Time Dependence on 1.5μm Photoluminescence of Laser-Ablated β-FeSi2
Authors: Uekusa Shin-ichiro , Aoki Kunitoshi , Hossain Mohammad Zakir , Fukuda Tomohiro , Miura Noboru
Published: April 2011

We prepared β-FeSi2 thin-films by using a Pulsed Laser Deposition (PLD) method and succeeded to observe photoluminescence (PL) around 1.5 μm corresponding to β-FeSi2 band from the long-time and high-temperature annealed β-FeSi2 thin-films. The β-FeSi2 thin-films were ablated on Si(111) substrates heated at 550°C. After ablation, long-time and high-temperature thermal annealing was performed in order to improve the crystal-quality. Annealing times were 5, 10, 20 and 40 hrs, and annealing temperature was kept at 900 °C. Crystallinity was evaluated by an X-ray diffraction (XRD) measurement. We have observed eminent improvement on crystal-quality of β-FeSi2 thin-films. Annealed samples show (220) or (202) X-ray diffraction signals of β-FeSi2 and the full width at half maximum (FWHM) of these peaks were 0.27° although the thickness of the samples decreased with annealing time. Thermal-diffusion of Si atoms was observed from substrate to thin-films. Fe atoms in the ablated thin-films also diffused into the substrate. The relationship between the thickness of β-FeSi2 thin-films and the thermal-diffusion were investigated with rutherford backscattering (RBS) measurement. Maximum photoluminescence intensity around 1.5 μm was observed from the thickest β-FeSi2 thin-film with only 5 hrs annealing.
Spectral and Temporal Resolution of THz Detectors based on Quantum Hall Devices with various Geometries
Authors: Kalugin Nikolai G. , Stellmach C. , Vasilyev Yu. , Hirsch A. , Hein G. , Sǎgol B. E. , Nachtwei G.
Published: April 2011

Quantum Hall (QH) systems can work as very sensitive THz detectors. In this work we report on the spectral and temporal resolution of the QH THz photodetectors with different geometries. The spectral resolution of THz QH detectors is of the order of 1-3 meV depending on bias conditions. The temporal resolution of QH detectors depends on the device geometry. For Corbino-shaped detectors we have demonstrated the devices with response times from 10 ns to over 200 ns.
Directional Growth of Si Nanowires on Insulating Films by Electric-Field-Assisted Metal-Induced Lateral Crystallization
Authors: Kanno Hiroshi , Kenjo Atsushi , Sadoh Taizoh , Miyao Masanobu
Published: April 2011

Metal-induced lateral crystallization of amorphous Si has been investigated under a wide range of electric fields (0-4000 V/cm). In the low field region (<100 V/cm), lateral growth velocity at the cathode side was enhanced by applying an electric field. This achieved formation of poly-Si with a large area (∼50 μm) during low-temperature annealing (525°C, 25 h). When the electric field exceeded 100 V/cm, the lateral growth velocity decreased with increasing the electric field strength. Under the extremely high electric field (>2000 V/cm), directional growth aligned to the electric field was observed. This new findings will be a powerful tool to achieve new poly-Si with highly controlled structures.
UV photoconductors based on Ga-doped ZnO films
Authors: Mandalapu Leelaprasanna J. , Xiu Faxian , Yang Zheng , Liu Jianlin
Published: April 2011

Ga-doped n-type ZnO films were grown by molecular-beam epitaxy (MBE) on R-plane sapphire substrates. Material characterizations such as photoluminescence (PL), X-ray diffraction (XRD), secondary ion mass spectroscopy (SIMS) and Hall measurements were carried out to characterize the optical, structural, and electrical properties of the film. Photoconductor devices for ultraviolet (UV) light detection were fabricated by depositing Al (250 nm)/Ti (20 nm) ohmic metal contacts. Linear characteristics were obtained from current-voltage (I-V) measurements that showed response to UV and visible illumination. Voltage dependent photocurrent (PC) spectra and responsivity spectrum were obtained to characterize the detection capability of the device in the UV region.
Nonmagnetic Doping Effect on the Magneto-Transport Properties of Mn Doped ZnO Dilute Magnetic Semiconductors
Authors: Mundada Govind , Manchiraju Srikanth , Kehl Ted , Pulugam Sandhya , Patel Rishi J. , Kahol Pawan , Ghosh Kartik
Published: April 2011

In this paper we report non magnetic elements (Al and Cu) doping effect on the structural and magneto-transport properties of Zn0.85Mn0.15O dilute magnetic semiconductors. Thin films of undoped Zn0.85Mn0.15O (ZnMnO), Al doped Zn0.80Al0.05Mn0.15O (ZnMnAlO), and Cu doped Zn0.85Cu0.05Mn0.15O (ZnMnCuO) were grown on sapphire single crystals using pulse laser deposition technique. X-Ray Diffraction and Raman Spectra confirm the epitaxial growth with a strong orientation along the c-axis. Scanning Electron Microscopy and Atomic Force Microscopy reveals surface microstructure of all the films. Magneto transport properties quantify the carrier concentrations and mobilities at room temperature in all the films.
Growth and Properties of Lattice Matched GaAsSbN Epilayer on GaAs for Solar Cell Applications
Authors: Bharatan Sudhakar , Iyer Shanthi , Matney Kevin , Collis Ward J. , Nunna Kalyan , Li Jia , Wu Liangjin , McGuire Kristopher , McNeil Laurie E.
Published: April 2011

In this work, the growth and characterization of GaAsSbN epilayers nearly lattice matched to GaAs, grown in an elemental solid source molecular beam epitaxy (MBE) system with a RF plasma nitrogen source, are discussed. The Sb and N compositions of the nearly lattice matched layers are 2.6% and 6.8%, respectively, as determined by high resolution x-ray diffraction (HRXRD) and secondary ion mass spectroscopy (SIMS) analysis. The layers are found to be fully strained as evidenced by the presence of Pendellosung fringes on the x-ray diffraction spectra.

Effects of in-situ and ex-situ annealing on the low temperature photoluminescence (PL) characteristics are discussed. The 10 K PL peak energy of 1 eV with a FWHM of 18 meV has been achieved on ex-situ annealed samples in N ambient. The temperature dependence of PL peak energy exhibits “S-shaped” behavior in the low temperature regime, indicative of the presence of localized excitons. Raman spectroscopy analysis has been carried out to determine the local structural changes on annealing.
White-light Emitting ZnO-SiO2 Nanocomposite Thin Films Prepared by Sputtering Method
Authors: Peng Yu-Yun , Hsieh Tsung-Eong , Hsu Chia-Hung
Published: April 2011

ZnO-SiO2 nanocomposite thin films were prepared using RF sputtering method without substrate heating. The photoluminance (PL) measurements showed that the ZnO-SiO2 nanocomposite thin films are able to emit white light consisting of violet, blue, and green-yellow band emissions. The blue emission was attributed to the large amount of ZnO/SiO2 interfaces, which enlarges the depletion layer width and then enhances the related transition. Further analyses indicated that the defect structure of samples could be manipulated by the amount and distribution of ZnO nanoparticles in SiO2 matrix to yield distinct luminescence property.
Minority Carrier Lifetime Measurement in Germanium on Silicon Heterostructures for Optoelectronic Applications
Authors: Sheng Josephine J. , Carroll Malcolm S.
Published: April 2011

Demand for low cost and high density near infrared (NIR) detection has motivated the development and use of germanium on silicon (Ge/Si) heterostructures to extend the optoelectronic application of Si technology. Ge/Si structures are currently being considered for NIR p/n detectors that can be integrated with Si CMOS [1]. Various research demonstrations of integrated Ge/Si diodes with CMOS have been made including using sputtered poly-Ge to form Ge/Si photodiodes after the CMOS transistors were complete. Poly-crystalline germanium (poly-Ge) may be formed by various methods including the use of plasma enhanced chemical vapor deposition. In this work, the formation of poly-Ge/Si heterostructures by inductively coupled plasma enhanced chemical vapor deposition (ICP-CVD) is examined as an alternative method to integrate poly-Ge into a CMOS process flow. In this work, 25 nm poly-Ge on Si heterostructures are formed by either recrystallization of ICP-CVD hydrogenated amorphous germanium (α-Ge:H) or direct deposition of ICP-CVD poly-Ge. A rapid measure of the suitability for detectors of the different poly-Ge films is the minority carrier recombination lifetime, which can affect dark current, quantum efficiency and overall detector detectivity. Recombination lifetimes were measured, therefore, in α-Ge:H that was recrystallized using rapid thermal annealing between 400 – 1050 °C in nitrogen ambient. Lifetimes were measured using a non-contact inductively coupled photo-conductance setup and an effective surface recombination velocity is subsequently extracted for each Ge/Si heterostructure that describes the integrated recombination in the Ge layer and at the Ge/Si interface, which separates the Ge contribution from recombination in the bulk silicon and at the silicon surface. The effective recombination velocities for the 25 nm Ge layers are found to be ∼103−104 cm/s.
Structural, Optical, and Magnetic Behavior of in-situ Doped, MOCVD-Grown Ga1-xMnxN Epilayers and Heterostructures
Authors: Li Nola , Fenwick William E. , Strassburg Martin , Asghar Ali , Gupta Shalini , Kang Hun , Li Nola , Summers Christopher , Ferguson Ian T.
Published: April 2011

Dilute magnetic semiconductors (DMS) show promise as materials that can exhibit ferromagnetism at room temperature (RT). However, the nature of ferromagnetism in this material system must be well understood in order to allow intelligent design of RT spintronic devices. This work investigates the magnetic properties of the as-grown films and the effect of Mn incorporation on crystal integrity and device performance. Ga1-xMnxN films were grown by MOCVD on c-plane sapphire substrates with varying thickness and Mn concentration. Homogenous Mn incorporation throughout the films was verified with Secondary Ion Mass

Spectroscopy (SIMS), and no macroscopic second phases (MnxNy) were detected using X-ray diffraction (XRD). Superior crystalline quality in the MOCVD-grown films relative to Mn-implanted GaN epilayers was confirmed via Raman spectroscopy. Vibrating sample magnetometry measurements showed an apparent room temperature ferromagnetic hysteresis in the as-grown epiayers. Similarly, a marked decrease in the magnetization was observed with annealing and silicon doping, as well as in post-growth annealed Mg-codoped samples. The observed decrease in muB per Mn with increasing Mn concentration is explained by Raman spectroscopy results, which show a decrease in long-range lattice ordering and an increase in nitrogen vacancy concentration with increasing Mn concentration. Magnetic and electron-spin paramagnetic resonance (EPR) data also show that the position of the Fermi level relative to the Mn2+/3+ level is the determining factor in magnetization. Light emitting diodes (LEDs) containing a Mn-doped active region have also been produced. Devices were fabricated with different Mn-doped active layer thicknesses, and I-V characteristics show that the devices become more resistive as thickness of the Mn-doped active layer increases. The magnetic and structural properties observed in this work will be used in conjunction with characteristics and magneto-optical of the Mn-containing devices to discuss the theoretical models of ferromagnetism in Ga1-xMnxN
InN Nano Rods and Epitaxial Layers Grown by HVPE on Sapphire Substrates and GaN, AlGaN, AlN Templates
Authors: Syrkin Alexander , Usikov Alexander , Soukhoveev Vitali , Kovalenkov Oleg , Ivantsov Vladimir , Dmitriev Vladimir , Collins Charless , Readinger Eric , Shmidt Natalia , Nikishin Sergey , Kuryatkov Vladimir , Song Daoying , Holtz Mark
Published: April 2011

This paper contains results on InN growth by Hydride Vapor Phase Epitaxy (HVPE) on various substrates including sapphire, GaN/sapphire, AlGaN/sapphire, and AlN/sapphire templates. The growth processes were carried out at atmospheric pressure in a hot wall reactor in the temperature range from 500 to 650°C. Arrays of nano-crystalline InN rods with various shapes were grown directly on sapphire substrates. Continuous InN layers were grown on GaN/sapphire, AlN/sapphire and AlGaN/sapphire template substrates. X-ray diffraction rocking curves for the (00.2) InN reflection exhibit the full width at half maximum (FWHM) as narrow as 0.075 deg. for the nano-rods and 0.128 deg. for the continuous layers grown on GaN/sapphire templates.
Analysis of high-power packages for white-light-emitting diode lamps with remote phosphor
Authors: Luo Hong , Kim Jong Kyu , Xi Yangang , Schubert E. Fred , Cho Jaehee , Sone Cheolsoo , Park Yongjo
Published: April 2011

An optimized packaging configuration for high-power white-light-emitting diode (LED) lamps that employs a diffuse reflector cup, a remote phosphor and a hemi-spherically-shaped encapsulation is presented. Ray tracing simulations for this configuration show that the phosphor efficiency can be enhanced by up to 50% over conventional packages. It is experimentally shown that dichromatic LED lamps with remote phosphor and diffuse reflector cup configuration have higher phosphor efficiency by 15.4% for blue-pumped yellow phosphor and by 27% for ultraviolet-pumped blue phosphor over conventional packages. Those improvements are attributed to reduced absorption of the phosphorescence by the LED chip and the reduction of deterministic optical modes trapped inside the encapsulant.
Fast High-Density Low-Pressure Plasma Synthesis of GaN Nanocrystals
Authors: Anthony Rebecca Joy , Thimsen Elijah , Johnson Joe , Campbell Stephen A , Kortshagen Uwe
Published: April 2011

Gallium Nitride is of interest due to its direct bandgap, which allows for efficient emission in the near-UV range. Bulk GaN is already in use in solid-state devices that exploit its emissive properties, however, the promise of GaN nanocrystals as tunable emitters for use in light-emitting devices and lasers has led to the recent exploration of nanocrystalline GaN synthesis routes. Here we discuss the use of nonthermal plasmas for the synthesis of nanocrystalline pow-ders of GaN. The particles were examined using transmission electron microscopy and x-ray photoelectron spectroscopy.
Opto-electronic Simulation of GaN Nanowire Lasers
Authors: Chen Liang , Towe Elias
Published: April 2011

A self-consistent, coupled opto-electronic simulation for studying GaN nanowire lasers is presented. The model solves, simultaneously and self-consistently, the carrier transport equations and the photon rate equations. The basic physical model takes into account both bulk and surface dark recombinations, stimulated emission, the anisotropic optical gain typical of the Wurtzite GaN structure, the modified spontaneous emission, and its coupling into the lasing modes by microcavity effects. The model further incorporates band gap shrinkage effects due to band renormalization and the effects of multiple lateral and longitudinal lasing and non-lasing optical modes.
Analysis of the Quantum Efficiency of GaInN/GaN Light Emitting Diodes in the Range of 390 - 580 nm
Authors: Zhao Wei , Li Yufeng , Xia Yong , Zhu Mingwei , Detchprohm Theeradetch , Schubert E. Fred , Wetzel Christian
Published: April 2011

In an attempt to identify the performance limiting factors of green GaInN/GaN light emitting diodes (LEDs) we analyze a large number of LED dies fabricated from over 160 epitaxy runs covering the wavelength range from 390 – 580 nm on a quantitative scale of the emission power. As a function of drive current, we analyze the external quantum efficiency (EQE) with particular emphasis on the low current range. We observe three very distinct populations of EQE behaviors. We find that the EQE maximum value has a strong correlation with the forward current in this point, while it has weak correspondences with both, the forward voltage in this point, and the dominant wavelength at 20 mA. The very good linear relationship between the forward voltage in green LED dies at 1 mA and at 20 mA is also observed. All those characteristics are meaningful traits to ultimately converge to a physical electronic transport and bandstructure model for the next step enhancement of green and deep green GaInN/GaN LED performance.
Application of Aluminum Nitride Thin Film for Micromachined Ultrasonic Transducers
Authors: Wang Qianghua , Xu Jianzeng , Huang Changhe , Auner Gregory W
Published: April 2011

This paper reports the fabrication and characterization of micromachined ultrasonic transducers (MUT) based on piezoelectric aluminum nitride (AlN) thin films. The MUT device is composed of an Al/AlN/Al sandwiched structure overlaid on top of a silicon (Si) diaphragm. X-ray diffraction (XRD) scan shows that highly c-axis oriented AlN (002) thin films have been grown on Al/Si(100) substrates. Electrical impedance of the MUT devices is analyzed as a function of frequency. The fundamental resonant frequencies of the devices are found in the range of 65-70 kHz, which are in approximation to the theoretical calculation. The effective coupling factors of the devices are also derived as 0.18.
Mechanism of Current Leakage in Ni Schottky Diodes on Cubic GaN and AlxGa1-xN Epilayers
Authors: As Donat J. , Potthast Stefan , Fernandez Jara , Lischka Klaus , Nagasawa Hiroyuki , Abe Masayuki
Published: April 2011

Ni Schottky-diodes (SDs) 300 μm in diameter were fabricated by thermal evaporation using contact lithography on cubic GaN and AlxGa1-xN epilayers. Phase-pure cubic GaN and c-Al0.3Ga0.7N/GaN structures were grown by plasma assisted molecular beam epitaxy (MBE) on 200 µm thick free-standing 3C-SiC (100) substrates. The quality of the cubic group III-nitride epilayers was checked by high resolution X-ray diffractometry, atomic force microscopy and photoluminescence at room temperature and at 2 K. Large deviations from the thermionic emission transport were observed in the current voltage (I-V) behavior of these SDs. Detailed analysis of the I-V characteristics at 300 K and at low temperature showed that a thin surface barrier is formed at the Ni semiconductor interface. Thermal annealing in air at 200°C alters the composition of this thin surface barrier and reduces the leakage current by three orders of magnitude. The doping density dependence of breakdown voltages derived from the reverse breakdown voltage characteristics of c-GaN SDs is in good agreement with theoretically calculated values and follows the expected trend. From these experimental data a blocking voltage of higher than 600V is extrapolated for c-GaN films with a doping level of ND = 5×1015 cm-3.
Simulation of Self-Heating and Temperature Effect in GaN-based Metal-Semiconductor Field-Effect Transistor
Authors: Turin Valentin O , Balandin Alexander
Published: April 2011

Two-dimensional electro-thermal simulations of GaN-based metal-semiconductor field-effect transistor are performed in the framework of the drift-diffusion model. The dependence of the hot spot temperature in transistors with many gates on the gate-to-gate pitch is studied. The case of SiC substrate is compared to the case of sapphire substrate. The ambient temperature effect on transistor performance is simulated. The specific of a thermal breakdown in GaN-based devices is discussed. The results obtained can be useful for the optimization of the thermal design for field-effect transistors.
Electrical domains and sub-millimeter signal generation in AlGaN/GaN superlattices
Authors: Gordion Irina , Manasson Alexander , Litvinov Vladimir
Published: April 2011

The paper discusses the feasibility of a terahertz-signal source made of AlGaN/GaN superlattice. Negative differential conductivity, electrical domain formation, current oscillations, and power efficiency of a perspective source are described. We relate the superlattice geometry and conduction band profile, distorted by polarization fields, to the oscillation frequency and power efficiency of the device. We also determine the optimal Al content, superlattice period, and the parameters of external circuit that favor sub-millimeter wave generation.
Design and Development of MBE Grown AlGaN/ GaN HEMT Devices on SiC Substrates for RF Applications
Authors: Sood Ashok K , Singh Rajwinder , Puri Yash R , Clarke Frederick W , Dabiran Amir , Chow Peter , Deng Jie , Hwang James C.M.
Published: April 2011

GaN /AlGaN transistors are being developed for a variety of RF electronic and high temperature electronics applications that will replace GaAs and Silicon based devices and amplifiers for commercial and military applications. In this paper, we present GaN/AlGaN based HEMT device architectures on SiC substrates with simulation and modeling results. The HEMT epitaxial layers were grown using RF Plasma Assisted MBE Technique. This approach has demonstrated very uniform epitaxial layers. The key to high performance HEMTs is the ability to grow high quality (Al)GaN buffer layers. Details of the electrical and optical characteristics of the HEMT wafers are presented. In addition, we will present results on an modified (ICP) etching technique that allows for low damage device processing and improved reliability.
Substrate influence on the high-temperature annealing behavior of GaN: Si vs sapphire
Authors: Pastor David , Cuscó Ramon , Artús Luis , Iborra Enrique , Jiménez Juan , Peiró Francisca , González-Díaz German , Calleja Enrique
Published: April 2011

We have studied the effects of rapid thermal annealing at 1300°C on GaN epilayers grown on AlN buffered Si(111) and on sapphire substrates. After annealing, the epilayers grown on Si display a stained surface. Scanning electron microscopy and optical confocal images revealed the presence of crater-shaped inhomogeneities that develop around protruding nuclei with tipically 50 micron diameter. Energy dispersive x-ray microanalysis yields a high concentration of Si in the crater regions as well as non-stoichiometric concentrations of Ga and N, with an excess of N.

Micro-Raman spectra obtained within the crater region exhibit Raman peaks associated with Si3N4 but no trace of GaN modes, while focused ion beam milling of this region leads to the accumulation of metallic Ga in the etched area. These results suggest that a substantial migration of Si from the substrate takes place during the annealing, which severely alters the material in the crater regions. Such annealing effects, which are not observed in GaN grown on sapphire, constitute a severe drawback for a widespread use of Si(111) substrates when high-temperature processing is required.
Charge Profiling of the p-AlGaN Electron Blocking Layer in AlGaInN Light Emitting Diode Structures
Authors: Xia Yong , Li Yufeng , Zhao Wei , Zhu Mingwei , Detchprohm Theeradetch , Schubert E. Fred , Wetzel Christian
Published: April 2011

Characterization of operational AlGaInN heterostructure light emitting diodes (LEDs) is critical to their performance optimization and time-to-failure analysis. Typically, device performance data needs to be corroborated with structural information such as layer thicknesses, charge profiles, and the absolute location of the pn-junction. Here, non-destructive testing by capacitance-voltage profiling is being applied to AlGaInN LED structures. Within a large set of samples with different active layer geometry, we observe distinct layers of high mobile charge accumulation. We correlate those with layer thicknesses derived from an x-ray diffraction analysis of the corresponding epiwafers. In this way, we identify the charge maxima as the upper and lower interfaces of the p-type AlGaN electron blocking layer to the neighboring GaN layers. By means of this successful analysis, we now have the opportunity to monitor epi process performance and stability as well as device degradation progress quasi-continuously over the device lifetime in a non-destructive mode.
Optical properties of GaN Photonic Crystal Membrane Nanocavities at Blue Wavelengths
Authors: Choi Yong-Seok , Meier Cedrik , Sharma Rajat , Hennessy Kevin , Haberer Elaine D. , Nakamura Shuji , Hu Evelyn L.
Published: April 2011

We have investigated the design parameters for high-Q photonic-crystal (PC) bandgap modes in the emission wavelengths of InGaN/GaN multiple quantum wells. We demonstrate experimental schemes to realize 2D triangular-lattice PC membrane structures, which is essential to obtain photonic bandgap (PBG) modes, and the optical properties of L7 membrane nanocavities that consist of seven missing holes in the Γ-K direction. L7 cavities show pronounced resonances with Q factors of 300 to 800 in the PBG as well as the enhancement of light extraction of the broad InGaN/GaN multiple-quantum-well emission by the 2D PBG.
Intersecting basal plane and prismatic stacking fault structures and their formation mechanisms in GaN
Authors: Bai Jie , Huang X. , Dudley M.
Published: April 2011

Comparative TEM studies have been carried out on GaN/AlN epifilms grown on both on-axis and off-cut 6H-SiC substrates to study the defects formed in the GaN/AlN films and the state of strain relaxation at the interface. Prismatic Stacking Faults (PSFs) are observed to form at I1 type substrate steps in both the on-axis and vicinal samples. In the vicinal samples, the PSFs expand into GaN/AlN film forming intersecting stacking fault configurations comprising faults that fold back and forth from the basal plane (I1 Basal-Plane Stacking Faults; BSFs) to the prismatic plane (PSFs). On the other hand, in the on-axis sample the PSFs are observed to mostly annihilate each other to form enclosed domains confined to the near-interface region. In addition, HRTEM studies suggest the existence of Geometric Partial Misfit Dislocations (GPMDs) at the SiC/AlN interface of the vicinal sample, which form at I2 type substrate steps. These GPMDs simultaneously accommodate the lattice mismatch and stacking sequence mismatch present at the SiC/AlN interface. This provides explanation of the improved strain relaxation observed in the vicinal versus the on-axis sample.
Oxygen Segregation to Nanopipes in Gallium Nitride
Authors: Hawkridge Michael E , Cherns David
Published: April 2011

This paper examines the core structure and composition of threading dislocations in GaN grown by hydride vapour phase epitaxy. Transmission electron microscopy showed that screw dislocations have widely varying core structures from open cores (“nanopipes”) to closed cores, with irregular variations between the two observed along the length of many dislocations. New evidence was found demonstrating that the equilibrium structure of screw dislocations is a closed core configuration. Electron energy loss spectroscopy combined with high resolution imaging showed that 1.7±0.3 monolayers of nitrogen were substituted by oxygen at the surfaces of nanopipes. In contrast, closed core dislocations showed little evidence of oxygen segregation. It is argued that these results support a model where nanopipe formation is controlled by the segregation of oxygen to pits predominately associated with, albeit not exclusively, dislocations. The implication of the results in understanding the electronic behavior of dislocations in GaN is also discussed.
Photoluminescence in wurtzite GaN containing carbon
Authors: Reshchikov Michael A. , Patillo Random H. , Travis Kathleen C.
Published: April 2011

We studied photoluminescence (PL) from a set of GaN layers grown on sapphire substrates by metalorganic chemical vapor deposition with the concentration of carbon varied by the growth conditions. One of the remarkable features in these samples is the extremely low intensity of the shallow donor-acceptor pair band. Analysis of the PL data gives the shallow acceptor concentration of less than 1014 cm-3 in most of the C-doped GaN layers. This result shows that C does not form a shallow acceptor, CN, in appreciable concentrations in wurtzite GaN. As for the YL band, there is no clear correlation between its intensity and the degree of C-doping. The question of identification of the deep acceptor responsible for the YL band in undoped and C-doped GaN still remains to be solved.
Characterization of the blue emission of Tm/Er co-implanted GaN
Authors: Roqan Iman , Trager-Cowan Carol , Hourahine Ben , Lorenz Katharina , Nogales Emilio , O’Donnell Kevin P , Martin Robert W , Alves Eduardo , Ruffenach S , Briot Olivier
Published: April 2011

Comparative studies have been carried out on the cathodoluminescence (CL) and photoluminescence (PL) properties of GaN implanted with Tm and GaN co-implanted with Tm and a low concentration of Er. Room temperature CL spectra were acquired in an electron probe microanalyser to investigate the rare earth emission. The room temperature CL intensity exhibits a strong dependence on the annealing temperature of the implanted samples. The results of CL temperature dependence are reported for blue emission (∼ 477 nm) which is due to intra 4f-shell electron transitions (1G4 → 3H6) associated with Tm3+ ions. The 477 nm blue CL emission is enhanced strongly as the annealing temperature increases up to 1200°C. Blue PL emission has also been observed from the sample annealed at 1200°C. To our knowledge, this is the first observation of blue PL emission from Tm implanted GaN samples. Intra-4f transitions from the 1D2 level (∼ 465 nm emission lines) of Tm3+ ions in GaN have been observed in GaN:Tm films at temperatures between 20–200 K. We will discuss the temperature dependent Tm3+ emission in both GaN:Tm,Er and GaN:Tm samples.