bluuebunny/crossref_metadata_2025 huggingface dataset:
Anomalous X-ray Scattering for Determination of Nanostructured Alloy Formation and Site-specific Chemistry of Bragg Peak
Authors: Chow G.M.
Published: April 2011

The properties of nanostructured films can be tailored by many factors such as composition, structure, preferred orientation, microstructure, interphase interface and grain boundaries. Compositional control of magnetic properties of recording media depends on the miscibility of constituent elements. However, for nanostructured materials with a large surface-to-volume ratio of atoms, the miscibility at the nanoscale may not necessarily follow the prediction of conventional phase diagram that does not consider the effects of surface and interface. This limitation further complicates the prediction of phase separation at grain boundaries and interphase interface. Using anomalous x-ray scattering, the alloying and composition of a specific long range order of textured nanostructured films has been investigated. Such information may not be readily available from conventional characterization techniques.
Mesoscale Thin Film Actuator for Promoting Fluid Motion in Microfluidic and Nanofluidic Channels
Authors: Sadler Daniel J. , Singh Gaurav , Zenhausern Frederic , Saraf Ravi F.
Published: April 2011

Microfluidic and nanofluidic devices often require actuators to induce fluid motion for applications such as pumping and mixing in small channels. Mixing, for instance, is important in systems where channel or chamber dimensions are on the order of 100 μm or larger as diffusive mixing can be prohibitively slow at these dimensions. In this work, a new mesoscale thin film polymer electromechanical actuator is introduced for use in the aforementioned applications. Unlike inorganic piezoelectric actuators, the devices based on these materials will be relatively easy to fabricate involving no high temperature processing, crystal growth, or microlithography. Fabrication of an array of actuators is simply achieved by spin casting the polymer over top of lithographically patterned gold electrodes at a thickness of less than 50 nm. This simple process enables a microfluidic device based on these actuators to be an integral part of a microfluidic channel rather than a separate unit operation. Depending on the application, the actuator array can be designed and controlled for random perturbations of the fluid flow field as required for mixing or for systematic actuation as required for pumping. These thin-film mesoscale actuators have been characterized and show extremely favorable properties such as a high electrostrictive response (compared to none in the bulk) and a frequency response of up to 50 kHz. In addition, finite element simulations show feasibility of these actuators for use in microfluidic mixing applications.
Vibrating RF MEMS for Low Power Communications
Authors: Nguyen Clark T.-C.
Published: April 2011

Micromechanical communication circuits fabricated via IC-compatible MEMS technologies and capable of low-loss filtering, mixing, switching, and frequency generation, are described with the intent to miniaturize wireless transceivers. Possible transceiver front-end architectures are then presented that use these micromechanical circuits in large quantities to substantially reduce power consumption. Technologies that integrate MEMS and transistor circuits into single-chip systems are then reviewed with an eye towards the possibility of single-chip communication transceivers.
Characterisation of the mechanical properties of MEMS devices using nanoscale techniques
Authors: Randall Nicholas X. , Soden Richard A. J.
Published: April 2011

This paper focuses on recent developments in the localised characterisation of the mechanical properties of Microsystems and MEMS devices and structures. Conventional indentation techniques provide a highly powerful method for measuring the load and depth response of bulk and coated materials, but can also be used to measure the mechanical properties of very small micro-machined silicon structures. Beam structures, such as are used for accelerometers, need to be characterised in terms of the number of cycles to failure, the spring constant or the energy required to bend the beam by a required amount. Such localised testing needs to be adapted to work at various distances from the origin of the beam with a positioning accuracy of less than a micron. Initial studies have proved to be highly repeatable. A range of examples is presented which covers a range of application areas, including accelerometer beam structures, microswitches and printer head structures. The basic instrumental concepts are explained together with the modifications required for testing small structures in a localised way.
Development of Fatigue Pre-Cracking Method into Micro-Sized Specimens for Measuring Fracture Toughness
Authors: Takashima K. , Koyama S. , Nakai K. , Higo Y.
Published: April 2011

In our previous investigations [1, 2], we have demonstrated that the introduction of fatigue pre-crack ahead of a notch is required to measure reliable fracture toughness values even for micro-sized specimens. However, it is rather difficult to introduce a fatigue pre-crack into a micro-sized specimen as once a fatigue crack starts to grow then the fatigue fracture occurs within one thousand cycles and this makes it extremely difficult to control fatigue crack length. Therefore, a new fatigue pre-cracking method is required for measuring fracture toughness. In this investigation, a new fatigue pre-cracking method has been proposed for micro-sized specimens and fracture toughness tests were carried out for the micro-sized specimens with fatigue pre-crack. Micro-cantilever beam type specimens with dimensions of 10 × 10 × 50 μm3 were prepared from an electroless deposited Ni-P amorphous alloy thin film and notches were introduced by focused ion beam machining. Fatigue pre-cracks were introduced ahead of the notches by far-field cyclic compression method using a mechanical testing machine for micro-sized specimens (MFT2000). Fracture tests were also carried out using the testing machine. Fatigue pre-cracks with length of 0.2 μm were confirmed on the fracture surfaces ahead of the notches in the far-field cyclically compressed specimens. This indicates that the fatigue pre-cracking method developed in this investigation is promising for measuring accurate fracture toughness for micro-sized specimens for MEMS applications.
Selective Titanium Oxide Formation Using Electron-beam Induced Carbon Deposition Technique
Authors: Djenizian Thierry , Macak Jan , Schmuki Patrik
Published: April 2011

The present work describes the selective titanium oxide formation based on electron-beam (e-beam) induced carbon deposition used as a mask against chemical dissolution. Under ideal conditions the C-deposits act as a negative resist to hinder the titanium oxide dissolution at e-beam treated locations. Carbon patterns were written in a scanning electron microscope at different electron doses on titanium oxide electrochemically grown in 1 M NaOH by potentiostatic experiments. Subsequently, the untreated areas were chemically removed in 0.5 % HF leaving the C-protected TiO2 patterns at the Ti surface. The selectivity of the technique depends on several factors such as the electron dose during masking and the chemical parameters. Under ideal conditions, it is demonstrated that direct patterning of surfaces as well as the fabrication of microstructures can be achieved using such technique.
Hybrid Integration of III-V Optoelectronic Devices on Si Platform Using BCB
Authors: Katsnelson Alex , Tokranov Vadim , Yakimov Michael , Lamberti Matthew , Oktyabrsky Serge
Published: April 2011

Integration of dense arrays of high frequency III-V photoemitters and photodetectors with Si platform is one of the challenging tasks for realization of novel chip-level optical interconnects. These interconnects require the resolution of numerous problems of compatibility of materials. Comparison of monolithic and hybrid integration technologies highlights the advantages of hybrid approaches for emitters highly sensitive to growth defects. A novel protocol for fabrication of III-V optoelectronic components on a Si platform is proposed. Reversed vertical cavity surface emitting laser (VCSEL) structures were grown homoepitaxialy by MBE on a GaAs substrate, and then bonded to a Si wafer using a benzocyclobutene (BCB) polymer. The GaAs substrate was subsequently removed by selective etching down to an AlAs etch stop layer. This reduces thermal stresses in order to enhance the optoelectronic device performance and increase lifetime. A 10 μm-thick high frequency VCSEL with coplanar metallization is processed on Si with PMGI reflow planarization. Electro-luminescence spectrum, I-V and P-T characteristics were measured and compared with a reference structure. It was found that measured thermal resistance is about five times higher than for devices on a host GaAs wafer.
Aluminum Nitride Chip Carrier for Micro-electro-mechanical Sensor Applications
Authors: Marinis T. F. , Soucy J. W.
Published: April 2011

A commercially fabricated aluminum nitride chip carrier was evaluated for packaging various types of MEMS inertial sensors. They were successfully assembled and vacuum-sealed within AlN chip carriers and their pressures have remained stable for over one year. Aging tests were conducted under electrical bias at 85°C and 85 %RH. The leakage currents were not as stable as those measured in alumina chip carriers and post test inspection of the AlN parts revealed etching of the ceramic between conductors.
Process Development and Integration of Piezoelectric Aluminum Nitride Thin-Film for RF MEMS Applications
Authors: Sharma Rajnish K. , Ning Jiang , Hanhua Feng , Gopalakrishnan R.
Published: April 2011

The aluminum nitride (AlN) process was successfully developed for as high frequency thin-film for bulk acoustic resonator applications. Columnar AlN was deposited on p-type (100) silicon substrate at 300–500°C using pulse direct current (DC) sputtering technique. Argon to nitrogen ratio was optimized to achieve stoichiometry at higher deposition rates of about 1000 Å/min at 1–4 kW DC power. The XRD of as-deposited and annealed AlN film shows preferred (002) orientations. The dry etching process was developed and etching rate about 3200 Å/min was achieved using Cl based chemistry. Film composition analysis was done using AES technique and stoichiometry was achieved.
Fabrication of Micromachined Piezoelectric Diaphragm Pumps Actuated by Interdigitated Transducer Electrodes
Authors: Hong Eunki , Krishnaswamy S.V. , Braggins T. T. , Freidhoff C.B. , Trolier-McKinstry S.
Published: April 2011

Micromachined pump structures were fabricated using surface micromachining. The structures consisted of three or five connected chambers. Sequential actuation of the diaphragms on these chambers will yield peristaltic pumping. As the actuation material, lead zirconate titanate (PZT [53/47] ) thin films were employed and actuated by interdigitated transducer (IDT) electrodes. First, chambers and channels on the 4″ (100) silicon wafers were defined by reactive ion etching (RIE) and thermally oxidized. The etched structures were then filled with amorphous silicon. Low temperature (silicon) oxide (LTO) and PZT films were then deposited. The LTO and PZT layers act as passive and active layers in a piezoelectric unimorph, respectively. A ZrO2 layer was employed to prevent reaction between SiO2 and PZT layers. A Cr/Au electrode was evaporated on top of the PZT layer and patterned into ring-shaped IDT electrodes. Finally, a porthole at each end of the pump structures was defined by ion milling and the whole micropump structure was released by removing the sacrificial amorphous silicon using XeF2. Completely released 500 μm diameter pump structures were fabricated. The remanent polarizations of the PZT films on released diaphragms were ∼20 μC/cm2 and their coercive fields were ∼50 kV/cm. 500 μm diameter diaphragms were deflected as much as 2 μm with 120 V applied. The shape and behavior of the diaphragm deflection can be explained by considering both d31 and d33 piezoelectric coefficients of the PZT films.
Mechanical Properties of Ultrananocrystalline Diamond Thin Films for MEMS Applications
Authors: Espinosa H.D. , Peng B. , Kim K.-H. , Prorok B.C. , Moldovan N. , Xiao X.C. , Gerbi J.E. , Birrell J. , Auciello O. , Carlisle J.A. , Gruen D.M. , Mancini D.C.
Published: April 2011

Microcantilever deflection and the membrane deflection experiment (MDE) were used to examine the elastic and fracture properties of ultrananocrystalline diamond (UNCD) thin films in relation to their application to microelectromechanical systems (MEMS). Freestanding microcantilevers and membranes were fabricated using standard MEMS fabrication techniques adapted to our UNCD film technology. Elastic moduli measured by both methods described above are in agreement, with the values being in the range 930 and 970 GPa with both techniques showing good reproducibility. The MDE test showed fracture strength to vary from 3.95 to 5.03 GPa when seeding was performed with ultrasonic agitation of nanosized particles.
Epitaxial silicon carbide simulations vs. experiments: etching, growth rates and aluminum/nitrogen doping
Authors: Mézière Jérôme , Blanquet Elisabeth , Pons Michel , Dedulle Jean-Marc , Ferret Pierre , Cioccio Léa Di , Billon Thierry
Published: April 2011

This paper summarizes recent experimental and simulation results on etching, growth rates and aluminum/nitrogen incorporation in SiC epitaxial layers grown in a horizontal LPCVD hotwall reactor commercialized by the Epigress company. The combined use of modeling and experiments allows to identify and to quantify the main growth phenomena. In this paper, a chemistry model including surface deposition and hydrogen etching is first described. It is found that the contribution of the etching of the susceptor to the SiC growth is not negligible. A simple model is used to describe nitrogen incorporation.
Reconstruction and Epitaxial Adlayers on SiC Surfaces: Structural Significance for Technological Applications
Authors: Starke U.
Published: April 2011

Structure and composition are reviewed for different surface phases of SiC(0001) and SiC(0001). The chemistry of well ordered phases spans from Si rich to graphitic composition with the excess elements arranged in adlayers and/or large scale reconstruction patterns. On SiC(0001) the Si rich (3×3) surface allows for step flow growth of monocrystalline homoepitaxial layers due to an extremely efficient dangling bond saturation. Si rich preparation or oxidative treatment are used for a controlled evolution of different stacking sequences on the adatom phase on SiC(0001) and could be utilized for the development of polytype heterostructures. On SiC(0001) a (2×2) phase shows three-fold coordinated adatoms with a different surface stacking reflecting the different growth behaviour of this surface. An epitaxially well matching silicon oxide monolayer can be prepared on both surface orientations by using a hydrogen etching or plasma treatment promising to facilitate low defect oxide films for MOS devices.
Growth of Columnar SiC on Patterned Si Substrates by CVD
Authors: Nishino Shigehiro , Okui Yoichi , Tai Yuehai , Jacob Chacko
Published: April 2011

Single crystalline 3C-SiC films have been grown on Si(111) substrate at 1200°C by conventional CVD process using HMDS (Hexamethyldisilane). Before columnar growth of SiC, columnar Si was made by depositing Au on Si(111) substrate as a solvent of VLS mechanism. Si growth was carried out by disproportional reaction in halide transport method. The columnar Si was produced on the patterned substrates. The columnar Si was covered by SiC. Needle like columns of SiC can be used for MEMS application such as micro-heat exchanger.
Characterization of Porous SiC Substrates and of the Epilayer Structures Grown on Them
Authors: Raghothamachar Balaji , Bai Jie , Vetter William M. , Gouma Perena , Dudley Michael , Mynbaeva Marina , Smith Matthew T. , Saddow Stephen E.
Published: April 2011

Porous 6H-SiC and 4H-SiC wafers formed by anodization have been characterized in this study prior to and following the CVD deposition of SiC epitaxial layers, using a combination of synchrotron white beam x-ray topography (SWBXT), SEM, TEM and optical microscopy. Under the high temperatures employed during epitaxial growth, a significant change in pore morphology occurs. While no evidence of reduced screw dislocation density in the epilayers is obtained, a small tilt of the epilayers with respect to the porous substrate observed on x-ray topographs could play a role in limiting penetration of defects from the substrate.
Spectroscopic Properties of Cubic SiC on Si
Authors: Feng Z. C. , Talwar D. N. , Ferguson I.
Published: April 2011

Spectroscopic studies are reported for cubic SiC grown on Si by chemical vapour deposition (CVD) manufacture technique. The UV excitation room temperature photoluminescence (PL)-Raman spectra exhibited 2.3 eV luminescence line due to RT recombination over the SiC indirect band gap. In addition to the optical phonons from cubic SiC we observed new Raman modes near 620 and 833 cm-1. A comprehensive analysis of the dynamical properties of defects using Green's function theory has suggested isolated impurity vibrations to be the possible origin for the additional phonons observed by Raman spectroscopy.
Self-assembled Ge quantum dots on SiC substrates grown by UHV-CVD
Authors: Calmes C. , Le V. , Bouchier D. , Saddow S. E. , Yam V. , Débarre D. , Laval R. , Clerc C.
Published: April 2011

We report our first results using a ultra high vacuum chemical vapor deposition (UHV-CVD) system to form Ge quantum dots on off-axis SiC substrates. Pure SiH4 and hydrogen-diluted GeH4 were used as gas precursors. The SiC substrates were chemically cleaned using the modified RCA process and the SiO2 layer was removed in-situ under a low SiH4 flow rate at a temperature between 1030°C and 1080°C. The Ge quantum dots were grown at a temperature of 750°C. In-situ reflection high-energy electron diffraction (RHEED) was used to monitor the surface cleaning and the Ge quantum dot growth. Ex-situ scanning electron microscope and atomic force microscopy were used to confirm the presence of Ge dots. The observed dots are smaller (350 Å width and 100 Å height) than similar Ge dots grown on Si.
SiC TO SiC WAFER BONDING
Authors: Yushin G. N. , Kvit A. V. , Collazo R. , Sitar Z.
Published: April 2011

SiC wafers with an RMS roughness of 1.5 nm were bonded in a dedicated ultrahigh vacuum bonding chamber. Successful fusion of wafers was observed at temperatures as low as 800°C under a uniaxial mechanical stress of 20 MPa. Cross-section transmission electron microscopy (XTEM) of a specimen bonded at 1100°C revealed parts of the interface where wafers were in intimate contact, while other parts contained an up to 3 nm thick amorphous carbon interlayer. The bonded SiC retained its high crystalline quality; no extended defects emanating from the interface were observed within the sampling region. Electrical measurements showed that the azimuthal orientation of the bonded couple significantly influences the electrical character of the junction.
Afterglow Thermal Oxidation of Silicon Carbide
Authors: Hoff Andrew M. , Tibrewala Arti , Saddow Stephen E.
Published: April 2011

We report, for the first time, the growth of thermal oxides on silicon carbide at temperatures below 900 °C using a novel afterglow thermal reactor. Flowing afterglows of microwave plasmas have been used for decades to study fundamental chemical reactions and in recent years they are employed as rich atomic oxygen sources for one of the principal techniques of polymeric photoresist removal in IC plants around the globe. The advantage of afterglow generation of a high flux of reactive species is that microwave plasmas may be excited inside clean quartz tubing and further the plasma potential of such discharges is less than 10 electron volts, which is far below the sputtering threshold of most materials, lowering the chance of contamination. In our case, we have integrated an afterglow source with a vacuum furnace system. The substrates to be oxidized are placed in the hot zone of the furnace where they are exposed to the flowing afterglow at a pressure near 1 Torr. The afterglow chemistry generated from pure oxygen traveling through the source includes O2, atomic oxygen, and excited states of O2. All ions and electrons generated by the plasma source are confined to the microwave cavity limits and photons are excluded by trapping techniques. The wafers in the furnace are exposed to a rich chemistry of afterglow species that were generated by non-thermal means. The furnace temperature alone controls the chemical reaction between the afterglow species and the substrates. We present the results of oxide growth on 4H and 6H silicon carbide substrates over a range of temperature. An example result is over 200Å of film grown at 800 °C in 2 hours. Atmospheric processing in O2 at 1050 °C would require 14 hours to grow a similar thickness on SiC. Possible mechanisms and oxide properties will also be discussed.
Transport Mechanisms in Focused Ion Beam Assisted Ohmic Contacts to p-Type 6H-SiC
Authors: Iliadis Agis A.
Published: April 2011

The current transport mechanism in non-annealed Ohmic contact metallizations on p-type 6H-SiC formed by using focused ion beam (FIB) surface-modification and direct-write metal deposition is reported, and the properties of such focused ion beam assisted non-annealed contacts are discussed. The process uses a Ga focused ion beam to modify the surface of the semiconductor with different doses, and then introduces an organometallic compound in the Ga ion beam, to effect the direct-write deposition of a metal on the modified surface. Contact resistance measurements by the transmission line method produced values in the low 10-4 Ω cm2 range for surface-modified and direct-write Pt and W non-annealed contacts, and mid 10-5 Ω cm2 range for surface-modified and pulse laser deposited TiN contacts. The current transport mechanism of these contacts was examined and found to proceed mainly by tunneling through the metal-modified-semiconductor interface layer.
Characterization of a Ceramic-Metal-Ceramic Bond: Chemical Vapor Deposited (CVD) Silicon Carbide Joined by a Silver-Based Active Brazing Alloy (ABA)
Authors: Marzik James V. , Oyama Toshi , MoberlyChan Warren J. , Croft William J.
Published: April 2011

Chemical vapor deposited (CVD) silicon carbide (SiC) ceramic material was joined to itself via an air stable, silver-based active brazing alloy (ABA). The microstructure and microchemistry of the interface was characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Results were compared to previous studies on the active alloy brazing of sintered silicon carbide using higher copper alloys.
Homoepitaxial 4H-SiC films grown by microwave plasma chemical vapor deposition
Authors: Okamoto Mitsuo , Kosugi Ryoji , Nakashima Shinichi , Fukuda Kenji , Arai Kazuo
Published: April 2011

Homoepitaxial 4H-SiC film growth has been carried out at temperatures as low as 1000°C on 4H-SiC of Si-face and C-face by microwave plasma chemical vapor deposition method. The extent of step-bunching of those films grown on C-face was low in comparison with that on Si-face, although large and irregular shaped step-bunching was occurred in both films grown on Si-face and C-face. For the first step to application for the electrical devices, the electrical properties of the μPCVD grown films was characterized by fabricating simple pn-junction structure. The obtained SiC films indicated n-type conductivity and the amount of background donor impurities of the films grown on C-face substrates were lower by one order than that on Si-face.
Aluminum and boron diffusion in 4H-SiC
Authors: Linnarsson M. K. , Janson M. S. , Schöner A. , Svensson B. G.
Published: April 2011

A brief survey is given of some recent result of boron diffusion in low doped n-type (intrinsic) and p-type 4H-SiC. Aluminum diffusion and solubility limit in 4H-SiC are also discussed. Ion implantation of boron has been performed in epitaxial material to form a diffusion source but also epitaxial 4H-SiC structures, with heavily boron or aluminum doped layers prepared by vapor phase epitaxy have been studied. Heat treatments have been made at temperatures ranging from 1100 to 2050°C for 5 minutes up to 64 h. Secondary ion mass spectrometry has been utilized for analysis. For boron diffusion in acceptor doped 4H-SiC, 4×1019 Al atoms/cm3, an activation energy of 5.3 eV has been determined and a strong dependence on Al content for the diffusion coefficient is revealed. Transient enhanced diffusion of ion-implanted boron in intrinsic 4H-SiC samples is discussed. Solubility limits of ∼1×1020 Al/cm3 (1700°C) and <1×1020 B/cm3 (1900°C) have been deduced.
Electrical Instability Suppression in 4H-SiC Power MESFETs
Authors: Tucker J. B. , Beaupre R. A. , Zhang A. P. , Garrett J. L. , Rowland L. B. , Kaminsky E. B. , Kretchmer J. W. , Vertiatchikh A. , Eastman L. F. , Allen A. F. , Edward B.
Published: April 2011

SiC has attracted great interest for high power microwave applications because of its superior intrinsic properties compared to Si and GaAs. Steady demonstrations of increasingly higher power handling capability have been achieved in recent years. However, SiC MESFETs still suffer from significant drain current degradation under RF operation or long-term DC stress. This degradation can be recovered after long periods of relaxation or immediately by illumination under UV light, which is indicative of a trapping effect. The origin of this effect has been attributed to either electron trapping at the device surface between the gate and drain or trapping at the epi-substrate interface due to the presence of electrically active contaminants in the bulk. Newly available “high purity” (non-vanadium compensated) bulk 4H semi-insulating SiC substrates were used in an effort to limit the effect of V-related deep level trapping at the substrate/epilayer interface. To investigate the effect of V on SiC MESFET performance, we compare similar devices fabricated on V compensated, and “high-purity” 4H-SiC substrates without intentional V doping. Presence or absence of V is confirmed by secondary ion mass spectrometry (SIMS) analysis. Pulsed I-V measurements as well as current- and capacitance-based deep level transient spectroscopy (DLTS) measurements were performed to assess trapping activation energy and density. An assessment of device performance and stability for each substrate type is made using RF load-pull measurements and device long-term DC bias stressing at temperature.
Influence of Interface States on High Temperature SiC Sensors and Electronics
Authors: Ghosh Ruby N. , Tobias Peter , Golding Brage
Published: April 2011

Silicon carbide based metal/oxide/semiconductor (MOS) devices are well suited for operation in chemically reactive high temperature ambients. The response of catalytic gate SiC MOS sensors to hydrogen-containing species has been assumed to be due to the formation of a dipole layer at the metal/oxide interface, which gives rise to a voltage translation of the high frequency capacitance voltage (C-V) curve. From in-situ C-V spectroscopy, performed in a controlled gaseous environment, we have discovered that high temperature (800 K) exposure to hydrogen results in (i) a flat band voltage occurring at a more negative bias than in oxygen and (ii) the transition from accumulation to inversion occurring over a relatively narrow voltage range. In oxygen, this transition is broadened indicating the creation of a large number of interface states. We interpret these results as arising from two independent phenomena – a chemically induced shift in the metal/semiconductor work function difference and the passivation/creation of charged states (DIT) at the SiO2/SiC interface. Our results are important for both chemical sensing and electronic applications. MOS capacitance gas sensors typically operate in constant capacitance mode. Since the slope of the C-V curve changes dramatically with gas exposure, we discuss how sensor-to-sensor reproducibility and device response time are influenced by the choice of operating point. For electronic applications understanding the environmentally induced changes in DIT is crucial to designing drift-free MOS devices. Our results are applicable to n-type SiC MOS devices in general, independent of the specifics of sample fabrication.
Cantilever Epitaxy of GaN on Sapphire: Further Reductions in Dislocation Density
Authors: Follstaedt D. M. , Provencio P. P. , Koleske D. D. , Mitchell C. C. , Allerman A. A. , Missert N. A. , Ashby C. I. H.
Published: April 2011

The density of vertical threading dislocations at the surface of GaN grown on sapphire by cantilever epitaxy has been reduced with two new approaches. First, narrow mesas (<1 μm wide) were used and {11–22} facets formed over them early in growth to redirect dislocations from vertical to horizontal. Cross-sectional transmission electron microscopy was used to demonstrate this redirection and to identify optimum growth and processing conditions. Second, a GaN nuc-leation layer with delayed 3D → 2D growth transition and inherently lower threading dislocation density was adapted to cantilever epitaxy. Several techniques show that a dislocation density of only 2–3×107/cm2 was achieved by combining these two approaches. We also suggest other developments of cantilever epitaxy for reducing dislocations in heteroepitaxial systems.
Electrical, Optical, Structural, and Analytical Properties of Very Pure GaN
Authors: Look D. C. , Sizelove J. R. , Jasinski J. , Liliental-Weber Z. , Saarinen K. , Park S. S. , Han J. H.
Published: April 2011

Present hydride vapor phase epitaxial growth of GaN on Al2O3 can produce material of very high quality, especially in regions of the crystal far from the substrate/epilayer interface. In the present study, we characterize a 248-μm-thick epilayer, which had been separated from its Al2O3 substrate and etched on top and bottom to produce flat surfaces. Temperature-dependent Hall-effect data have been fitted to give the following parameters: mobility μ(300) = 1320 cm2/V-s; μ(peak) = 12,000 cm2/V-s; carrier concentration n(300) = 6.27 × 1015 cm−3; donor concentration ND = 7.8 × 1015 cm−3; acceptor concentration NA = 1.3 × 1015 cm−3; and effective donor activation energy ED = 28.1 meV. These mobilities are the highest ever reported in GaN, and the acceptor concentration, the lowest. Positron annihilation measurements give a Ga vacancy concentration very close to NA, showing that the dominant acceptors are likely native defects. Secondary ion mass spectroscopic measurements show that ND is probably composed of the common donors O and Si, with [O] > [S1]. Transmission electron microscopy measurements yield threading dislocation densities of about 1 × 107 cm−2 on the bottom (N) face, and < 5 × 105 cm−2 on the top (Ga) face. Photoluminescence (PL) spectra show a strong donor-bound exciton (D°X) line at 3.47225 eV, and a weaker one at 3.47305 eV; each has a linewidth of about 0.4 meV. In the two-electron satellite region, a strong line appears at 3.44686 eV, and a weaker one at 3.44792 eV. If the two strong lines represent the same donor, then ED,n=1 – ED,n=2 = 25.4 meV for that donor, and the ground-state activation energy (EC – ED,n=1) is (4/3)25.4 = 33.9 meV in a hydrogenic model, and 32.7 meV in a somewhat modified model. The measured Hall-effect donor energy, 28.1 meV, is smaller than the PL donor energy, as is nearly always found in semiconductors. We show that the difference in the Hall and PL donor energies can be explained by donor-band conduction via overlapping donor excited states, and the effects of non-overlapping excited states which should be included in the n vs. T data analysis (charge balance equation).
Hollow core dislocations in Mg-doped AlGaN
Authors: Cherns D. , Wang Y. Q. , Liu R. , Ponce F. A. , Amano H. , Akasaki I.
Published: April 2011

Transmission electron microscopy has been used to investigate the core structure of threading dislocations in heavily Mg-doped (1020 cm−3) Al0.03Ga0.97N films grown on (0001) sapphire by metal-organic chemical vapour deposition. Evidence is presented that Mg segregates to edge and mixed dislocations, and that these dislocations often have open cores with diameters in the range 1–5nm. The mechanism of hollow core formation and the role of Mg are discussed.
Measurements of the Refractive Indices of MOCVD and HVPE Grown AlGaN Films Using Prism-Coupling Techniques Correlated with Spectroscopic Reflection/Transmission Analysisa
Authors: Sanford Norman A. , Robins Lawrence H. , Davydov Albert V. , Shapiro Alexander J. , Tsvetkov Denis V. , Dmitriev Vladimir A. , Spring Silver , Keller Stacia , Mishra Umesh K. , DenBaars Steven P.
Published: April 2011

Waveguide prism-coupling methods were used to measure the ordinary and extraordinary refractive indices of AlxGa1-xN films grown on sapphire substrates by HVPE and MOCVD. Several discrete wavelengths ranging from 442 nm to 1064 nm were used and the results were fit to one-term Sellmeier equations. The maximum standard uncertainty in the refractive index measurements was ± 0.005 and the maximum standard uncertainty in the self-consistent calculation for film thickness was ± 15 nm. Analysis of normal-incidence spectroscopic transmittance and reflectance measurements, correlated with the prism-coupling results, was used to determine the ordinary refractive index as a continuous function of wavelength from the band gap wavelength of each sample (between 252 nm and 364 nm) to 2500 nm. The Al compositions of the samples were determined using energy-dispersive X-ray spectroscopy analysis (EDS). HVPE grown samples had compositions x = 0.279, 0.363, 0.593, and 0.657. MOCVD samples had x = 0.00, 0.419, 0.507, 0.618, 0.660, and 0.666. The maximum standard uncertainty in the absolute EDS-determined value for x was ± 0.02.
Excitons bound to surface defects in GaN
Authors: Reshchikov M. A. , Huang D. , Morkoç H.
Published: April 2011

Sharp intense peaks are sometimes detected in the low-temperature photoluminescence (PL) spectrum of undoped GaN samples in the photon energy range of 3.0 – 3.46 eV. Some of these peaks can be attributed to excitons bound to dislocations and inversion domains, whereas some others originate from the GaN surface because they can be affected essentially by surface treatment. In our samples, grown by molecular beam epitaxy on sapphire substrate, the 3.42 eV peak always disappeared after removing the surface layer by etching for a few seconds in hot phosphoric acid. Atomic force microscopy images confirmed that such light etching modifies the surface morphology, although the etched depth is negligibly small. Moreover, intensities of two other peaks (at 3.32 and 3.35 eV) were observed to depend on sample etching, as well as on the length of subsequent exposure to air. The 3.32 and 3.35 eV peaks evolved with time of UV illumination, increasing by several times and demonstrating memory effect at low temperature. We attribute the 3.42 and 3.35 eV peaks to bound excitons, whereas the 3.32 eV peak is tentatively attributed to a surface donor-acceptor pair transition.
Electron Stimulated Desorption of Deuterium from GaN (0001) Surface
Authors: Yang Y. , Lee J. , Thoms B. D.
Published: April 2011

Temperature programmed desorption (TPD) was performed on deuterated GaN(0001) surfaces which had been exposed to various doses of 90-eV electrons. TPD of the deuterated surface without electron exposure shows a broad D2 desorption feature with a peak desorption temperature at ∼400 °C. Electron exposure results in a decrease in intensity of the desorption peak which is attributed to removal of surface deuterium by electron stimulated desorption (ESD). This removal of deuterium by ESD produces no change in the peak desorption temperature indicating that recombinative desorption is first order in deuterium coverage.
Dislocation-related deep states induced by irradiation in HVPE n-GaN
Authors: Castaldini A. , Cavallini A. , Polenta L.
Published: April 2011

This paper deals with defects induced by proton irradiation in n-GaN. The samples were HVPE grown, irradiated with 24 GeV protons. DLTS was performed on both as-grown and irradiated samples with planar and normal collector configurations to evidence the dislocation effect. Two electron traps were identified in the as-grown material by both diode structures: trap EC1 (EC – 0.19eV) and trap EC2 (EC – 0.25eV), EC2 being the dominant one. Irradiation consistently affects the pre-existing levels in such a way so as their appearance strongly depends on the diode structure, hence on the region probed by DLTS.

This contribution focuses on the actual existence of two deep traps EC1 and EC2 emitting in GaN at low temperature and on the different nature of the deep levels associated to EC1 and EC2. Their filling kinetics was studied since we supposed that EC2 would be associated to extended defects, as already reported in literature. Indeed, the site density of EC2 logarithmically depends on the filling pulse width, demonstrating that this trap is definitely associated to extended defects. The trap EC1, on the contrary, exhibits the filling kinetics peculiar of point defects.
Surface Passivation of AlGaN terminated and GaN Terminated HEMT Structures Studied by XPS
Authors: Gila B. P. , Lambers E. , Luo B. , Onstine A. H. , Allums K. K. , Abernathy C. R. , Ren F. , Pearton S. J.
Published: April 2011

Samples of both AlGaN and GaN terminated HEMT structures were studied using x-ray photo spectroscopy (XPS). It was found that the XPS spectra of both AlGaN and native oxide surfaces were shifted by a surface charge of 0.5 to 1.0 eV. The samples were then oxidized using an UV-ozone treatment for 25 minutes at room temperature. The ozone oxide XPS spectra of the AlGaN terminated surface was found to have the same 0.5 to 1.0 eV shift while the ozone oxide XPS spectra of the GaN terminated surface was found to have a 5.0 eV shift, indicating that the native GaN oxide is more insulating. Processed HEMT devices using both surface terminations were given the same UV-ozone treatment followed by a 10nm MBE grown Sc2O3 film for passivation. The GaN terminated HEMT structures showed on average a 10% increase in channel current (from gate-lag measurements, pulse mode) over the AlGaN terminated HEMT structures.
Stable Ohmic Contacts on GaAs and GaN Devices for High Temperatures
Authors: Piotrowska A. , Kaminska E. , Barcz A. , Golaszewska K. , Wrzesinska H. , Piotrowski T. T. , Dynowska E. , Jakiela R.
Published: April 2011

We have studied thermal stability of Nb and NbN contacts to GaAs and GaN by x-ray diffraction and SIMS, and demonstrated their excellent behaviour under high temperature annealing. GaAs/Nb and GaAs/NbN contacts are stable up to 800°C and 900°C, respectively while GaN/NbN and GaN/Nb/NbN remain stable up to 1000°C.
Contacts to High Aluminum Fraction p-type Aluminum Gallium Nitride
Authors: Hull Brett A. , Mohney Suzanne E. , Chowdhury Uttiya , Dupuis Russell D. , Gotthold David , Birkhahn Ronald , Pophristic Milan
Published: April 2011

Gold, palladium, platinum or nickel ohmic contacts on Mg doped p-type AlxGa1-xN with x = 0.4 and x = 0.45 have been examined. The Au contact provided the lowest contact resistivity with pc = 1.8 (± 1.1) x 10−3 Ωcm2, but only following annealing at 850°C. For the Pd, Au, and Pt contacts annealed at greater than 700°C, a rapid degradation in the current-voltage curves was observed upon testing. The degradation was induced by exposure to sub-bandgap light and was reversed with a mild anneal at 500°C. Possible mechanisms for the degradation are discussed.
Properties of Surface States on GaN and Related Compounds and Their Passivation by Dielectric Films
Authors: Hasegawa Hideki , Hashizume Tamotsu
Published: April 2011

This paper reviews the authors′ recent efforts to clarify the properties of electronic states near surfaces of GaN and AlGaN by using variousin-situandex-situcharacterization techniques, including UHV contact-less C-V, photoluminescence surface state spectroscopy (PLS3), cathode luminescence in-depth spectroscopy (CLIS),and gateless FET techniques that have been developed by the authors’ group.

As a result, a model including a U-shaped surface state continuum, having a particular charge neutrality level, combined with frequent appearance of near-surface N-vacancy related deep donor states having a discrete level at Ec - 0.37eV is proposed as a unified model that can explain large gate leakage currents and current collapse in AlGaN/GaN HFETs. Hydrogen plasma treatment and SiO2deposition increase N-vacancy related deep donors. Reasonably good surface passivation can be achieved by ECR-plasma SiNx films and by ECR-plasma oxidized Al2O3films both combined with ECR N2plasma treatment.
Growth of Oriented Gallium Nitride Films on Amorphous Substrates by Self Assembly
Authors: Li Hongwei , Sunkara Mahendra K.
Published: April 2011

C-plane oriented thin films of gallium nitride (GaN) were grown on both amorphous quartz substrates and single crystalline c-sapphire substrates at sub-atmospheric pressures by exposing molten gallium thin films to electron cyclotron resonance (ECR) microwave generated nitrogen plasma. Gallium nitride crystals nucleated from molten gallium and self-aligned with respect to each other due to the mobility of nitrogenated gallium and formed textured film directly on amorphous substrates. Scanning electron microscopy (SEM) images and X-ray Diffraction (XRD) spectra confirmed the orientation among crystals. Micro-Raman spectra exhibited a FWHM of 3 cm−1. Self-assembled, nanocrystalline GaN thin films were obtained when spin-coated gallium thin films (< 1 μm) on quartz substrates were nitrided.
Raman Mapping and Finite Element Analysis of Epitaxial Lateral Overgrown GaN on Sapphire Substrates
Authors: Benyoucef M. , Kuball M. , Beaumont B. , Bousquet V. , Gibart P.
Published: April 2011

Using micro-Raman scattering and finite element (FE) analysis, stress fields in epitaxial lateral overgrown (ELO) GaN fabricated by metalorganic vapor phase epitaxy (MOVPE) on sapphire substrates using a two-step growth method were investigated. Nearly full stress relaxation at the top ELO GaN surface can be achieved by increasing the thickness of ELO GaN to about 50 μm. Reductions in stress variation between window and overgrown regions can be achieved by using a double ELO GaN growth at a much smaller ELO thickness. Increased compressive stress at the coalescence boundary of two adjacent wings of ELO GaN was related to the presence of voids in this area. In the double ELO growth, stress near the top surface was mainly attributed to the presence of voids on top of the upper dielectric mask.
Single Crystalline InN Films Grown on Si Substrates By Using A Brief Substrate Nitridation Process
Authors: Yamaguchi Tomohiro , Mizuo Kazuhiro , Saito Yoshiki , Noguchi Takuma , Araki Tsutomu , Nanishi Yasushi , Miyajima Takao , Kudo Yoshihiro
Published: April 2011

InN films were grown on Si (111) substrates by radio-frequency plasma-excited molecular beam epitaxy. InN films highly oriented to the c-axis were obtained by optimizing growth conditions in the direct growth on Si. Growth of single crystalline InN films was realized on Si substrates with substrate nitridation for 3 min. On the other hands, when the substrate nitridation was lasted over 30 min, obtained InN films were polycrystalline due to the amorphous SiNx layer formed on a substrate surface. We also studied the local atomic structure in the single crystalline InN film using extended X-ray absorption fine structure measurements.
The Influence of Substrate Surface Polarity on Optical Properties of GaN Grown on Single Crystal Bulk AlN
Authors: Tamulaitis G. , Yilmaz I. , Shur M. S. , Gaska R. , Chen C. , Yang J. , Kuokstis E. , Khan A. , Rojo J. C. , Schowalter L. J.
Published: April 2011

Photoluminescence of the GaN layers grown both on N-face and Al-face bulk AlN is studied under CW and pulsed laser excitation in the temperature range from 8 K to 300 K. We compare localization of excitons, residual strain, and activation energies for thermally activated transfer of carriers to nonradiative recombination. At high excitation intensities, conditions for carrier heating, which is important for the threshold of stimulated emission, are also investigated.
Optical Characterization of Bulk GaN Grown from a Na/Ga Flux
Authors: Palle K. , Chen L. , Liu H. X. , Skromme B. J. , Yamane H. , Aoki M. , Hoffman C. B. , DiSalvo F. J.
Published: April 2011

Bulk GaN crystals up to several mm in size, grown by a Na/Ga flux method, have been characterized using room and low temperature photoluminescence (PL) and panchromatic cathodoluminescence (CL) imaging. Highly resolved excitonic PL spectra are obtained for material grown in a new, large-scale reactor. The crystal polarity affects the incorporation of residual Zn and Mg or Si acceptors and the deep level luminescence bands in c-oriented platelets. A Zn (A°,X) triplet structure with unusual thermalization properties and a highly resolved structural defect related PL peak are observed. Striations are found in some of the smaller platelets by CL imaging, but are absent in the prismatic crystals.
Electroreflectance and Photoreflectance Studies of Electric Fields in Pt/GaN Schottky Diodes and AlGaN/GaN Heterostructures
Authors: Shokhovets S. , Goldhahn R. , Gobsch G. , Ambacher O. , Smorchkova I. P. , Speck J. S. , Mishra U. , Link A. , Hermann M. , Eickhoff M.
Published: April 2011

We have performed electroreflectance and photoreflectance studies of Pt/GaN Schottky diodes with Ga- and N-face polarity as well as AlGaN/GaN based transistor heterostructures. The experimental data were analyzed using electric field-dependent dielectric functions of GaN and AlGaN. Inhomogeneities in the electric fields were taken into account by application of a multi-layer formalism. We observed an increase of the electric field strength underneath the Schottky contact and in the AlGaN barrier with increasing temperature. The results are explained in terms of temperature dependent densities of ionized impurities and surface charges.
Effect of Oxygen Pressure on Magnesium Oxide Dielectrics Grown on Gan by Plasma Assisted Gas Source Molecular Beam Epitaxy
Authors: Onstine A. H. , Gila B. P. , Kim J. , Stodilka D. , Allums K. , Abernathy C. R. , Ren F. , Pearton S. J.
Published: April 2011

The effect of oxygen pressure on MgO grown by RF plasma assisted gas-source molecular beam epitaxy was investigated. Increasing oxygen pressure was found to decrease the growth rate, improve the morphology and reduce the Mg/O ratio to near that obtained from bulk single crystal MgO. By contrast, the electrical characteristics of MgO/GaN diodes showed continual improvement in breakdown field and interface state density as the pressure was decreased. The lowest pressure tested, 1×10−5Torr, produced the lowest Dit, 3×1011 eV−1cm−2, and the highest VBD, 4.4 MV/cm. Cross sectional transmission electron microscopy of the MgO grown at the lowest pressure showed the initial 40 monolayers to be epitaxial, with the remainder of the layer appearing to be fine grained poly-crystal. Comparisons with films grown using an electron cyclotron resonance (ECR) plasma suggest that higher ion energies are desirable for obtaining the best electrical characteristics.
High-Mobility Ga-Polarity GaN achieved by NH3-MBE
Authors: Wang J. X. , Wang X. L. , Sun D. Z. , Li J. M. , Zeng Y. P. , Hu G. X. , Liu H. X. , Lin L. Y.
Published: April 2011

GaN epilayers were grown on (0001) sapphire substrates by NH3-MBE and RF-MBE (radio frequency plasma). The polarities of the epilayers were investigated by in-situ RHEED, chemical solution etching and AFM surface examination. By using a RF-MBE grown GaN layer as template to deposit GaN epilayer by NH3-MBE method, we found that not only Ga-polarity GaN films were repeatedly obtained, but also the electron mobility of these Ga-polarity films was significantly improved with a best value of 290 cm2/V.s at room temperature. Experimental results show it is an easy and stable way for growth of high quality Ga-polarity GaN films.