bluuebunny/crossref_metadata_2025 huggingface dataset:
A Pretreatment Technique to Improvement the Ashing Resistance of Low K Spin-On-Polymer (SOP)
Authors: Chang Kow-Ming , Yang Ji-Yi , Chang Yu-Hsun
Published: April 2011

The oxygen plasma via resists strip process cause significant damage to organic SOP, thus limiting its inter-level dielectric application. A simple treatment technology using reactive ion is proposed to reform the SOP surface. The reactive ion modification of the SOP can improve the resistance towards oxygen plasma. This is owing to the carbon atom absence in the SOP's surface area. The measurements of Fourier transform infrared (FTIR) spectroscopy, x-ray photoelectron spectroscopy (XPS), stress, thickness variation, Scanning Electron Microscope (SEM) cross-sectional view for gap filling and dielectric constant show that SOP with reactive ion treatment (RIT) has better quality for non-etch-back process than SOP without RIT.
Structire, Properties, and Process Characteristics of Low-K Materials Prepared by PECVD
Authors: Shimogaki Y. , Lim S. W. , Loh E. G. , Nakano Y. , Tada K. , Komiyama H.
Published: April 2011

Low dielectric constant F-doped silicon oxide films (SiO:F) can be prepared by adding fluorine source, like as CF4 to the conventional PECVD processes. We could obtain SiO:F films with dielectric constant as low as 2.6 from the reaction mixture of SiH4/N2 O/CF4. The structural changes of the oxides were sensitively detected by Raman spectroscopy. The three-fold ring and network structure of the silicon oxides were selectively decreased by adding fluorine into the film. These structural changes contribute to the decrease ionic polarization of the film, but it was not the major factor for the low dielectric constant. The addition of fluorine was very effective to eliminate the Si-OH in the film and the disappearance of the Si-OH was the key factor to obtain low dielectric constant. A kinetic analysis of the process was also performed to investigate the reaction mechanism. We focused on the effect of gas flow rate, i.e. the residence time of the precursors in the reactor, on growth rate and step coverage of SiO:F films. It revealed that there exists two species to form SiO:F films. One is the reactive species which contributes to increase the growth rate and the other one is the less reactive species which contributes to have uniform step coverage. The same approach was made on the PECVD process to produce low-k C:F films from C2F4, and we found ionic species is the main precursor to form C:F films.
Porous Organosilicates for On-Chip Dielectric Applications
Authors: Miller R. D. , Beyers R. , Carter K. R. , Cook R. F. , Harbison M. , Hawker C. J. , Hedrick J. L. , Lee V. , Liniger E. , Nguyen C. , Remenar J. , Sherwood M. , Trollsås M. , Volksen W. , Yoon D. Y.
Published: April 2011

Porous organosilicates useful for on-chip insulator applications can be prepared by templating the vitrification of low molecular weight silsesquioxanes (SSQs) using highly branched, thermally labile macromolecules which are subsequently removed in a thermal process to generate porosity. The process involves spin coating a mixture of the matrix material and the porogen (pore generator) followed by thermal curing to initiate vitrification and decomposition of the porogen. The morphology is fixed during the formation of the nanoscopic inorganicorganic hybrid and is maintained during foaming. This process generates controllable and stable morphologies where the void volume is determined by the porogen loading level. The porous materials are thermally robust and intrinsically hydrophobic without subsequent chemical treatment. Dielectric constants of < 2.2 are easily achieved for pore volumes of only 20%, and this porosity appears to be predominately closed cell in nature. These materials display a number of thermal mechanical and electric properties consistent with the requirements for on-chip insulator applications.
Porous Silicon Oxynitride Films Derived from Polysilazane as a Novel Low-Dielectric Constant Material
Authors: Aoki T. , Shimizu Y , Kikkawa T.
Published: April 2011

A novel spin on material derived from perhydropolysilazane that converts into ultra-low k inorganic films is described in this paper. The obtained films, cured at 400°C in N2 atmosphere, exhibit dielectric constants as low as 1.6 which do not change after holding the wafers in a clean-room mbient for 2 months. Cross-sectional SEM images of the cured films show the aggregation of small granules with diameters ranging from 5 to 30 nm. The films can be obtained by conventional SOG process: spin-coating, baking and curing, without any additional process such as hydrophobic treatment.

The average atomic compositions of the films are, Si/O/N/C = 40/55/5/0.5 (atomic %), by XPS analysis. These results indicate that the films have hydrogen silicon oxynitride structures. No evolution of H2O and NH3 was detected by TDS analysis in the temperature range of RT to 800°C. Hydrophobic Si-H and Si-H2 groups remaining in the film might prevent water absorption, resulting in the low dielectric constant.

The remainder of Si-H and Si-H2 constituents in the cured films is the result of selective oxidation reactions of perhydropolysilazane in the baking process with the use of a specific catalyst. The structures of the films are tailored by altering the amount of the catalyst. In this study, we also demonstrate the relationship between the effect of the catalyst and the film properties.
Structure-Property Correlation in Low K Dielectric Materials
Authors: Morgen Michael , Zhao Jie-Hua , Hay Michael , Cho Taiheui , Ho Paul S.
Published: April 2011

In recent years there have been widespread efforts to identify low dielectric constant materials that can satisfy a number of diverse performance requirements necessary for successful integration into IC devices. This has led to extensive efforts to develop low k materials and the associated process integration. A particularly difficult challenge for material development has been to find the combination of low dielectric constant and good thermal and mechanical stability. In this paper recent characterization results for low k materials performed at the University of Texas will be reviewed, with an emphasis on the relationship of chemical structure to the aforementioned key material properties. For example, measurements showing the effect of film porosity on dielectric constant and thermal and mechanical properties is presented. This data, as well as that for other material types, demonstrates the tradeoffs between dielectric constant and thermomechanical properties that are often made during the course of material development.
Thermal conductivity study of porous low K dielectric materials
Authors: Hu Chuan , Morgen Michael , Ho Paul S. , Jain Anurag , Gill William. N. , Plawsky Joel L. , Wayner Peter C.
Published: April 2011

A quantitative characterization of the thermal properties is required to assess the thermal performance of low dielectric constant materials. Recently we have developed a technique based on the 3-omega method for measuring the thermal conductivity of porous dielectric thin films. In this paper we present the results on the measurements of thermal conductivity of thin porous films using this method. A finite element method analysis is used to evaluate the approximations used in the measurement. Two porosity-weighted thermal resistor models are proposed to interpret the results. By studying the dependence of the thermal conductivity on porosity, we are able to discuss the scaling rule of thermal conductivity. Additionally, a steady state layered heater model is used for evaluating the significance of introducing porous ILDs into an interconnect structure.
ELectrical Characterization of Beryllium Doped Low Temperature MBE Grown GaAs
Authors: Cich M. J. , Zhao R. , Park Y. , Specht P. , Weber E. R.
Published: April 2011

The electrical properties of low temperature MBE grown GaAs (LT-GaAs) in the n-i-n configuration have been studied. The mechanism of current rise in beryllium doped LT-GaAs is found to fit Frenkel-Poole type emission with a barrier height of 0.26 eV. However, this model does not fit undoped LT-GaAs. The breakdown field is considerably higher (up to 5.2* 105 V/cm) for beryllium doped films than undoped films, and depends on both growth temperature and beryllium concentration. Beryllium doping is also found to increase the resistivity of the preannealed films to values > 109 ω-cm.
Temperature Dependent Photoluminescence Study of High Silicon doped AIGaAs and the Correlation between the Photoluminescence Spectra and the Doping Level
Authors: Koley G. , Liu W. , Prasad K. , Lin M.
Published: April 2011

We report on the room temperature and the low temperature ( 77 K ) study of highly Si doped MBE grown bulk Al.2Ga.8As layers by Photoluminescence ( PL ), Hall, X-Ray and Electrochemical Voltage Profiler ( ECV ) technique. The room temperature PL spectra shows the presence of an asymmetric broad peak in the low energy side whose Full Width at Half Maxima (FWHM) and intensity increases with the doping level. The broad peak has been found to be a combination of two individual peaks, one arising out of the DX center related recombinations and the other due to some Silicon related complex acceptor sites. The low temperature PL also shows a ‘strange peak’ near the Al2Gas8As band edge luminescence peak for wafers grown at 600°C. This peak which increases in intensity at a very rapid rate with falling temperature and shift very rapidly with temperature can be attributed to some AIGaAs growth defect related recombination center, depending on the growth temperature.
In Situ Observation of scaling Behavior During Solution-Phase Growth of Surfactant Monolayers
Authors: Schwartz Daniel K. , Doudevski Ivo
Published: April 2011

“Self-assembled” monolayers of amphiphilic surfactant molecules form spontaneously on solid surfaces by exposure to dilute solutions of the adsorbate molecules. These monolayers are shown to form via a mechanism that includes nucleation, growth, coalescence, etc. of densely- packed submonolayer islands of the long-chain organic molecules. In situ atomic force microscopy experiments allow a quantitative analysis of island nucleation and growth rates as well as determination of the island size distribution as a function of coverage. In the growth regime, the nucleation and growth rates have a power law behavior consistent with a simple point island model of 2D cluster growth. The exponents are consistent with a critical nucleus of two molecules and the 2D diffusion coefficient corresponds to a “hopping time” of about 1 microsecond. In the aggregation regime, the island size distributions are shown to scale with a single evolving length scale in accordance with the dynamical scaling approximation.
Preparation and Optical Properties of Ge and C-Induced Ge Quantum Dots on Si
Authors: Eberl K. , Schmidt O. G. , Kienzle O. , Ernst F.
Published: April 2011

Pure Ge epitaxially grown on Si (100) at high temperatures forms typically 100 nm lateral size islands on top of a 3–4 monolayer thick wetting layer. In stacked layers of Ge dots pronounced vertical alignment is observed if the thickness of the Si spacer layers is smaller than about 50 nm. Pregrowth of a small amount of C on Si substrate induces very small 10 nm size Ge quantum dots after deposition of about 2 to 3 monolayers Ge. Photoluminescence (PL) studies indicate a spatially indirect radiative recombination mechanism with the no-phonon line strongly dominating. Strong confinement shift in the 1–2 nm high and 1Onm lateral size dots results in low activation energies of 30 meV, which causes luminescence quenching above 50K.

For large stacked Ge islands with 13 nm thin Si spacer layers we observe a significantly enhanced Ge dot-related PL signal up to room temperature at 1.55μm wave length. This is attributed to a spatially indirect transition between heavy holes confined within the compressively strained Ge dots and two-fold degenerated Δ state electrons in the tensile strained Si between the Ge stacked dots.
Computer Simulations of Two-Dimensional Sn-Cu Alloys on (100) and (111) Cu Surfaces
Authors: Aguilar Jose F. , Ravelo R. , Baskes M.
Published: April 2011

We have performed calculations of Sn deposition on Cu(111) and Cu(100) surfaces. The atomic interactions are described by modified embedded atom method (MEAM) potentials. This is a modification of the embedded atom method (EAM) to include higher moments in the electron density. We find the at low coverages Sn deposited on Cu(111) leads to the formation of a two-dimensional (2D) alloy phase with a p (√3 × √3)-R 30° structure which is stable up to temperatures of 900K. These results are in agreement with ion-scattering experiments of thin films of Sn on Cu(111). For deposition of Sn on Cu(100), a 0.25 monolayer (ML) coverage results in the formation of a stable 2D alloy phase with a p(2 × 2) structure. This result is also in agreement with LEED measurements.
The Growth of Homo-Epitaxial Silicon at Low Temperatures Using Hot Wire Chemical Vapor Deposition
Authors: Thiesen J. , Jones K.M. , Matson R. , Reedy R. , Iwaniczko E. , Mahan H. , Crandall R.
Published: April 2011

We report on the first known growth of high-quality epitaxial Si via the hot wire chemical vapor deposition (HWCVD) method. This method yields epitaxial Si at the comparatively low temperatures of 195° to 450°C, and relatively high growth rates of 3 to 20 Å/sec. Layers up to 4500-Å thick have been grown. These epitaxial layers have been characterized by transmission electron microscopy (TEM), indicating large regions of nearly perfect atomic registration. Electron channeling patterns (ECPs) generated on a scanning electron microscope (SEM) have been used to characterize, as well as optimize the growth process. Electron beam induced current (EBIC) characterization has also been performed, indicating defect densities as low as 8×104/cm2. Secondary ion beam mass spectrometry (SIMS) data shows that these layers have reasonable impurity levels within the constraints of our current deposition system. Both n and p-type layers were grown, and p/n diodes have been fabricated.
Epitaxial Growth of Zinc-Blende Ain by Plasma Source Molecular Beam Epitaxy
Authors: Thompson Margarita P. , Auner Gregory W. , Drews Andrew R. , Zheleva Tsvetanka S. , Jones Kenneth A.
Published: April 2011

Epitaxial zinc-blende AIN films as thick as 2000Å were deposited on Si (100) substrates by plasma source molecular beam epitaxy (PSMBE). The metastable zinc-blende form of AIN was observed to occur when pulse d.c. power was supplied to the PSMBE hollow cathode source. Reflection High Energy Electron Diffraction (RHEED) showed that the films possess a four fold symmetry. X-Ray Diffraction (XRD) revealed two strong peaks corresponding to the (200) and (400) reflections from the zinc-blende AIN. The lattice parameter of the films was calculated to be approximately 4.373Å. TEM, performed on one of the films, revealed that the AIN is cubic, single crystalline and epitaxial with respect to the Si (100) substrate.
Mesoscopic Models of Sintering of Metallic Clusters. A Study by Molecular Dynamics Simulations
Authors: Mazzone A.M.
Published: April 2011

Motivated by the ulbiquitous role of clusters in film growth, we study coalescence of metallic clusters (Ag.Cu.Pb). of size from 50 to 300, with an isothermal molecular dynamics simulation method using classical forces. A comparison is made between the melting behavior of the isolated clusters, onl one side. and classical theories of sintering, on the other side. An evident failure of the macroscopic models appears from this comparison and this casts considerable doubts on the adequacy of continuum models for low dimensional systems.
Smoothing Surfaces by an AC Field: An Application of the Ratchet Effect
Authors: Lee C.-S. , Derényi I. , Barabási A.-L.
Published: April 2011

We demonstrate that for surfaces that have a nonzero Schwoebel barrier, the application of an ac field parallel to the surface will induce a net electromigration current. Most important, the direction of the net current will be always downhill; i.e. it will point in the step-down direction. The magnitude of this equilibrium current is calculated analytically, and compared with Monte Carlo simulations. A downhill current is known to smooth the surface, thus we suggest that the application of ac fields during annealing might aid the smoothing process and during growth it has the potential to slow or eliminate the Schwoebel barrier induced mound formation
Roughening of Au(111) Surfaces During Ion Beam Erosion: A Scanning Tunneling Microscope and X-Ray Diffraction Study
Authors: Judy A. , Murty M.V. Ramana , Butler E. , Pomeroy J. , Cooper B.H. , Woll A.R. , Brock J.D. , Kycia S. , Headrick R.L.
Published: April 2011

Using Scanning Tunneling Microscopy(STM) and X-ray diffraction(XRD), we have studied the development of surface roughness on Au(111) during 500eV Ar+ ion irradiation at different angles. During normal incidence erosion the surface roughens and pattern formation occurs. The surface morphology is a mixture of mounds and pits superimposed onto a larger structure of channels and valleys. The characteristic spacing between features grows with a power law behavior t27, where t is the amount of time the sample was irradiated, in agreement with previous measurements[l]. At glancing angles, erosion proceeds smoothly, but not in layer-by-layer fashion. Finally, a combination of glancing angle and normal incidence erosion is used to create a rippled morphology
Annealing Effects of Schottky Contacts on the Characteristics of 4H-SiC Schottky Barrier Diodes
Authors: Kang S. C. , Kum B. H. , Do S. J. , Je J. H. , SHIN M. W.
Published: April 2011

This paper reports on the relationship between the microstructure and the device performance of Pt/4H-SiC schottky barrier diodes ( SBDs ). The evolution of microstructure in the metal/SiC interfaces annealed at different temperatures was characterized using X-ray scattering techniques. The reverse characteristics of the devices were degraded with annealing temperatures. The maximum breakdown voltages of as-deposited devices and 850 °C annealed devices are 1300 V and 626 V, respectively. However, the forward characteristics of the devices were found out to improve with annealing temperatures. X-ray scattering analysis showed that Pt-silicides were formed by annealing performed at or higher than 650 °C. The formation of silicides was shown to increase the roughness of the Pt/SiC interface. It is believed that the forward characteristics of the SBDs be strongly dependent on the crystallity of silicides formed in the Pt/SiC interface during the annealing process.
Epitaxial Growth of SiC in a Vertical Multi-Wafer CVD System: Already Suited as Production Process?
Authors: Rupp Roland , Hecht Christian , Wiedenhofer Arno , Stephani Dietrich
Published: April 2011

Results about a new CVD system suited for epitaxial growth on six 2 inch SiC-wafers at a time are presented. Excellent gas flow stability is achieved for this new reactor type as shown by in- situ observations of the gas flow dynamics in the reactor chamber. These experimental results agree favorably with numerical process simulation results.

The epitaxial layers grown in the multi-wafer system so far show a by an order of magnitude higher background impurity level (≤1015 cm−3) as reported previously for layers grown in single-wafer systems by the authors and other groups (≤ 1014 cm−3). On the other hand, the doping homogeneity achieved until today is very encouraging. The variation on a 2 inch wafer is less than ± 20% at about 1*1016 cm−3. The wafer to wafer variation of the average doping value both within a run and from run to run is within 15 %. The reproducibility and uniformity of the layer thickness is even better (total thickness variation ≤5% on a 2 inch wafer). The surface of the epitaxial layers is very smooth with a typical growth step height of 0.5 nm (4H, 8° off orientation). First measurements on Schottky diodes build on these layers show low leakage current values indicating low point defect density in the epitaxial layers.
Multi-Wafer VPE Growth of Highly Uniform SiC Epitaxial Layers
Authors: O'Loughlin M. J. , Nordby H. D. , Burk A. A.
Published: April 2011

A multi-wafer silicon carbide vapor phase epitaxy reactor is employed that features full planetary motion and is capable of high quality epitaxy on seven, two-inch diameter substrates. We are currently performing preproduction growths of static induction transistor (SIT) and metal semiconductor field effect transistor (MESFET) active layers. On a 2-inch diameter substrate, layer uniformity is typically ±5% (standard deviation/mean) for both dopant concentration and layer thickness (for 1 3/8-inch substrates, layer uniformity is around ±3%). For the seven wafers within a run, interwafer uniformity has been dramatically improved to approximately ±8% for dopant concentration and ±3% for layer thickness. Process control charts will be presented which exhibit that interrun (run-to-run) variation in both thickness and doping can be kept within ±10% of the desired values.
3C-SiC Buffer Layers Converted from Si at a Low Temperature
Authors: Liaw H. M. , Hong S. Q. , Fejes P. , Werho D. , Tompkins H. , Zollner S. , Wilson S. R. , Linthicum K. J. , Davis R. F.
Published: April 2011

We have obtained single-crystal 3C-SiC films via conversion of the surface region of Si (111) and (100) wafers at 970 °C by reaction with C2H4 in an MBE reactor. The major defects in the films were clusters, voids, and misfit dislocations. Investigation by high resolution TEM images showed low lattice strains in the epitaxial layer due to the formation of 1 misfit dislocation for every 4 to 5 regular SiC planes that are bonded to Si at the interface. The clusters and voids often occurred in pairs. A model for forming the void-cluster pairs is suggested.
Time Resolved Photoluminescence of Cubic Mg Doped GaN
Authors: Seitz R. , Gaspar C. , Monteiro T. , Pereira E. , Schoettker B. , Frey T. , As D. J. , Schikora D. , Lischka K.
Published: April 2011

Mg doped cubic GaN layers were studied by steady state and time resolved photoluminescence. The blue emission due to Mg doping can be decomposed in three bands. The decay curves and the spectral shift with time delays indicates donor-acceptor pair behaviour. This can be confirmed by excitation density dependent measurements. Furthermore temperature dependent analysis shows that the three emissions have one impurity in common. We propose that this is an acceptor level related to the Mg incorporation and the three deep donor levels are due to compensation effects.
Dielectric Function of AlN Grown on Si (111) by MBE
Authors: Zollner Stefan , Konkar Atul , Gregory R. B. , Wilson S. R. , Nikishin S. A. , Temkin H.
Published: April 2011

We measured the ellipsometric response from 0.7–5.4 eV of c-axis oriented AlN on Si (111) grown by molecular beam epitaxy. We determine the film thicknesses and find that for our AlN the refractive index is about 5–10% lower than in bulk AlN single crystals. Most likely, this discrepancy is due to a low film density (compared to bulk AlN), based on measurements using Rutherford backscattering. The films were also characterized using atomic force microscopy and x-ray diffraction to study the growth morphology. We find that AlN can be grown on Si (111) without buffer layers resulting in truely two-dimensional growth, low surface roughness, and relatively narrow x-ray peak widths.
Mid Infrared Photoconductivity Spectra of Donor Impurities in Hexagonal Silicon Carbide
Authors: Linville R. J. , Brown G. J. , Mitchel W. C. , Saxler A. , Perrin R.
Published: April 2011

Mid-infrared photoconductivity (PC) is a useful technique for identifying and investigating donor and acceptor centers in many semiconductors. This is especially true when the PC results are combined with other measurements such as Hall Effect and DLTS. We report on the first Fourier Transform Infrared (FTIR) photoconductivity spectra for n-type 6H and 4H-SiC. The samples studied had temperature dependent Hall activation energies around 45 meV and 85 meV in the 4H samples, and a single activation energy of 106 meV in the 6H. For the 4H samples, the PC spectra showed an increase in photoresponse between 40 and 47 meV, with another sharp increase at 120 meV. In the 6H-SiC, the photoresponse also had a rapid increase at 120 meV, and at 77 meV in one sample. The photoresponse spectra of the n-type 4H and 6H-SiC samples were distinctly different in the mid-infrared.
SiC Power Electronic Devices, MOSFETs and Rectifiers
Authors: Cooper J. A. , Ryu S-H. , LI Y. , Matin M. , Spitz J. , Morisesette D. T. , McGlothlin H. M. , Das M. K. , Melloch M. R. , Capano M. A. , Woodall J. M.
Published: April 2011

SiC power switching devices have demonstrated performance figures of merit far beyond the silicon theoretical limits, but much work remains before commercial production will be feasible. A significant fraction of the remaining work centers on materials science issues. This paper reviews the current status of unipolar power switching devices in SiC and identifies the major technological and material science barriers that need to be overcome.
Comparative Study of Emission from Highly Excited (In, Al) GaN Thin Films and Heterostructures
Authors: Little B. D. , Bidnyk S. , Schmidt T. J. , Lam J. B. , Kwon Y. H. , Song J. J. , Keller S. , Mishra U. K. , DenBaars S. P. , Yang W.
Published: April 2011

The optical properties of (In, Al) GaN thin films and heterostructures have been compared under the conditions of strong nanosecond excitation. The stimulated emission (SE) threshold from AIGaN epilayers was found to increase with increasing Al content compared to GaN, in contrast to InGaN epilayers, where an order of magnitude decrease is observed. Optically pumped SE has been observed from AIGaN films with aluminum concentrations as high as 26%. Room temperature SE at wavelengths as low as 327 nm has been achieved. In contrast to the increase of SE threshold seen for AlGaN films, we found that AlGaN/GaN heterostructures which utilize carrier confinement and optical waveguiding drastically enhance the lasing characteristics. We demonstrate that AIGaN/GaN heterostructures are suitable for the development of deep ultraviolet laser diodes.
TEM Study of Mg-Doped Bulk GaN Crystals
Authors: Liliental-Weber Z. , Benamara M. , Ruvimov S. , Mazur J. H. , Washburn J. , Grzegory I. , Porowski S.
Published: April 2011

Transmission electron microscopy was applied to cross-sectioned samples to study surface morphology, sample polarity and defect distribution in bulk GaN samples doped with Mg. These crystals were grown from a Ga melt under high hydrostatic pressure of Nitrogen. It was shown that the types of defects and their distribution along the c-axis depends strongly on sample polarity. Based on this finding the growth rate along the c-axis for the two polar directions was compared and shown to be approximately ten times larger for Ga polarity than for N-polarity. In the part of the crystals with Ga polarity pyramidal defects with a base consisting of high energy stacking faults were found. The parts of the crystals grown with Npolarity were either defect free or contained regularly spaced stacking faults. Growth of these regularly spaced cubic monolayers is polarity dependent; this structure was formed only for the growth with N polarity and only for the crystals doped with Mg. Formation of this superstructure is similar to the polytypoid structure formed in AlN crystals rich in oxygen. It is also likely that oxygen can decorate the cubic monolayers and compensate Mg. This newly observed structure may shed light on the difficulties of p-doping in GaN:Mg.
Piezoelectric Coefficients of Aluminum Nitride and Gallium Nitride
Authors: Lueng C. M. , Chan H. L. W. , Fong W. K. , Surya C. , Choy C. L.
Published: April 2011

Aluminum nitride (AlN) and gallium nitride (GaN) thin films have potential uses in high temperature, high frequency (e.g. microwave) acoustic devices. In this work, the piezoelectric coefficients of wurtzite AlN and GaN/AlN composite film grown on silicon substrates by molecular beam epitaxy were measured by a Mach-Zehnder type heterodyne interferometer. The effects of the substrate on the measured coefficients are discussed.
Epitaxial Growth of AlN on Si Substrates with Intermediate 3C-SiC as Buffer Layers
Authors: Hong S. Q. , Liaw H. M. , Linthicum K. , Davis R. F. , Fejes P. , Zollner S. , Kottke M. , Wilson S. R.
Published: April 2011

Single crystalline AlN was successfully grown on a 3C-SiC coated Si (111) substrate by organometallic vapor phase epitaxy. The 3C-SiC film was obtained via the conversion of the Si near-surface region to SiC using gas-source molecular beam epitaxy. The quality of the AlN was mainly controlled by that of the SiC. The effects of Si pits and SiC hillocks formed during the conversion on subsequent AlN growth are discussed. Process optimization is suggested to improve the SiC buffer layer for subsequent AlN deposition.
The Influence of Spontaneous and Piezoelectric Polarization on Novel AlGaN/GaN/InGaN Device Structures
Authors: Foutz B. E. , Murphy M. J. , Ambacher O. , Tilak V. , Smart J. A. , Shealy J. R. , Schaff W. J. , Eastman L. F.
Published: April 2011

The strong spontaneous polarization and piezoelectric effects in the wurtzite IIInitride semiconductors lead to new possibilities for device design. In typical heterojunction field effect transistors these effects are used to create large electron concentrations at the AlGaN/GaN interface. However, we examine several other possible device structures which include heterojunctions of AlGaN, GaN, and InGaN. For example, we find the strong electric fields present in these structures allow us to create quantum wells greater than 1 eV deep. Both Ga-faced and N-faced materials are explored. The two-dimensional electron gas concentrations in these structures are found using a self-consistent 1-D Schrödinger-Poisson solver modified to incorporate the effects of spontaneous and piezoelectric polarization. The boundary conditions at the heterojunction interfaces and at the surface and substrate are discussed in detail. Electron concentrations are compared with those obtained experimentally through capacitance-voltage and Hall effect measurements.
Effect of Varying Oxidation Parameters on the Generation of C-Dangling Bond Centers in Oxidized SiC
Authors: Macfarlane P. J. , Zvanut M. E.
Published: April 2011

SiC is perhaps the most appropriate material to replace Si in power-metal-oxidesemiconductor- field-effect-transistors (MOSFETs), because, unlike the other wide band-gap semiconductors, SiC can be thermally oxidized similarly to Si to form a SiO2 insulating layer. In our studies of oxidized SiC, we have used electron paramagnetic resonance (EPR) to identify Cdangling bonds generated by hydrogen release from C-H bonds. While hydrogen's effect on SiCbased MOSFETs is uncertain, studies of Si-based MOSFETs indicate that it is important to minimize hydrogen in MOS structures. To examine the role of hydrogen, we have studied the effects of SiC/SiO2 fabrication on the density of C-related centers, which are made EPR active by a dry heat-treatment. Here we examine the starting and ending procedures of our oxidation routine. The parameter that appears to have the greatest effect on center density is the ending step of our oxidation procedure. For example, samples that were removed from the furnace in flowing O2 produced the smallest concentration of centers after dry heat-treatment. We report on the details of these experiments and use our results to suggest an oxidation procedure that limits center production.
Activation Characteristics of Donor and Acceptor Implants in GaN
Authors: Cao X. A. , Pearton S. J. , Singh R. K. , Wilson R. G. , Sekhar J. A. , Zolper J. C. , Han J. , Rieger D. J. , Shul R. J. , Guo H. J. , Pennycook S. J. , Zavada J. M.
Published: April 2011

The ionization levels of different donor and acceptor species implanted into GaN were measured by temperature-dependent Hall data after high temperature (1400 °C) annealing. The values obtained were 28 meV (Si), 48 meV (S), 50 meV (Te) for the donors, and 170 meV for Mg acceptor. P-type conductivity was not achieved with either Be or C implantation. Basically all of the implanted species show no distribution during activation annealing. For high implant doses (5×1015 cm−2) a high concentration of extended defects remains after 1100 °C anneals, but higher temperatures (1400 °C) produces a significant improvement in crystalline quality in the implanted region.
Transmutation Doping of III-Nitrides
Authors: Popovici Galina
Published: April 2011

Transmutation doping of (In, Ga, Al)N compounds by neutron irradiation is a promising and totally unexplored field to date. It is much more effective than that of Si due to large neutron capture cross section and abundance of In, Al and Ga isotopes participating in reaction. This should make the irradiation possible in low-flux reactors and result in smaller radiation damage. Annealing of the radiation damage seems feasible.
Bias-Temperature-Stress Induced Mobility Improvement in 4H-SiC MOSFETs
Authors: Chattyt K. , Chowt T. P. , Gutmannt R. J. , Arnoldi E. , Alok D.
Published: April 2011

In this work, we report on an instability which affects the field effect mobility in 4HSiC MOSFETs. The devices (MOSFETs and capacitors) were subjected to a biastemperature stress (BTS) for 30 minutes at 150°C at stress voltages corresponding to oxide fields upto 1MV/cm. Following a positive BTS(i.e. gate voltage positive), the field effect mobility increased by upto two orders of magnitude from the original value; upon application of a negative BTS to the MOSFET, the device characteristics degraded to the unstressed state. The high mobility state could be recovered by a positive BTS and was reversible with repeated bias stressing. An explanation of this phenomenon is proposed based on the effect of interfacial ions on the dependence of both trapped charge and inversion charge densities on gate bias.
A Materials Investigation of Nickel Based Contacts to n-SiC Subjected to Operational Thermal Stresses Characteristic of High Power Switching
Authors: Cole M. W. , Hubbard C. W. , Fountzoulas C. G. , Demaree D. J. , Ren F.
Published: April 2011

This study developed and performed Laboratory experiments which mimic the acute cyclic thermal loading characteristic of pulsed power device switching operation. Ni contacts to n-SiC were the device components selected for cyclic thermal testing. Modifications of the contact- SIC materials properties in response to cyclic thermal fatigue were quantitatively assessed via Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM), atomic force microscopy (AFM), surface profilometry, transmission electron microscopy (TEM), nanoindentation testing and current-voltage measurements. Decreases in nanohardness and elastic modulus were observed in response to thermal fatigue. No compositional modifications were observed at the metal-semiconductor interface. Our results demonstrated that the majority of the material changes were initiated after the first thermal pulse and that the effects of subsequent thermal cycling (up to 10 pulses) were negligible. The stability of the metalsemiconductor interface after exposure to repeated pulsed thermal cycling lends support for the utilization of Ni as a contact metallization for pulsed power switching applications.
Structure of Single-Crystal Gd2O3Films on GaAs(100)
Authors: Kortan A. R. , Hong M. , Kwo J. , Mannaerts J. P. , Kopylov N.
Published: April 2011

We have studied the single-crystal Gd2O3films grown epitaxially on GaAs(100) substrate with single-crystal x-ray diffraction. The sesquioxide Gd2O3forms two hexagonal phases, one monoclinic and one cubic phase in bulk form. In our studies of different thickness films, we have found that the Gd2O3grows only in the cubic phase with a unique epitaxial orientation. The two-fold (110) planes of the Gd2O3are oriented parallel to the four-fold GaAs(100) surface, while alligning its [001] and [110] axes with the [011] and [011] axes of GaAs within the plane, respectively. The film chooses only one of the two such possible orientations, which can be explained by the local bonding configuration at the interface. We find evidence for an elastic strain in the films less than 50 A thick.
MBE Growth of Oxides for III–N MOSFETs
Authors: Gila B. , Lee K N. , Laroche J , Ren F , Donovan S. M. , Abernathy C. R. , Han J.
Published: April 2011

Reproducible fabrication of high performance metal oxide semiconductor field effect transistors (MOSFETs) from compound semiconductors will require both good interfacial electrical characteristics and good thermal stability. While dielectrics such as SiO2, AIN, and GdGaOx have demonstrated low to moderate interface state densities, questions remain about their thermal stability and reliability, particularly for use in high power or high temperature widebandgap devices. In this paper we will compare the utility of two potential gate dielectric materials: GdOx and GaOx. GdOx has been found to produce layers with excellent surface morphologies as evidenced by surface roughness of less than I nm. Stoichiometric films can be easily obtained over a range of deposition conditions, though deposition temperatures of 500°C appear to offer the optimum interfacial electrical quality. By contrast GaOx films are quite rough, polycrystalline and show poor thermal stability. Further they exhibit a range of stoichiometries depending upon deposition temperature, Ga flux and oxygen flux. This paper will describe the relationship between deposition conditions and film characteristics for both materials, and will present electrical characterization of capacitors fabricated from GdOx on Si.
Interface State Densities for SiNx: H on Cleaved GaAs and InP(110)
Authors: Landheer D. , Hulse J. E. , Rajesh K.
Published: April 2011

Silicon nitride was deposited in-situ by electron-cyclotron resonance plasma chemical-vapour deposition (ECR-CVD) on (110) surfaces formed by cleaving GaAs and InP(100) substrates in an ultra-high vacuum processing system. Capacitors formed by depositing Al gates on the facet surfaces were analyzed by the high-low frequency capacitance-voltage (CV) technique. The minimum interface-state densities obtained for the cleaved GaAs (110) surfaces were 1–2 × 1012 eV−1cm−2. For cleaved InP facets the measured minimum interface state densities were a factor of two higher; however, they exhibited a smaller hysteresis in the CV characteristics and a smaller modulation in the surface potential. The interface state densities did not change significantly for the GaAs(110) facets if a Si interface control layer 0.8–2 nm thick was deposited prior to silicon nitride deposition; however, a larger effect was observed for the hysteresis and flatband voltage shift of the CV characteristics. The effect of annealing on the interfaces with Si was investigated and the performance compared with published results for GaAs(100) surfaces prepared by molecular-beam epitaxy.
Effect on Performance of Composition of Li-ion carbon Anodes derived from PMAN/DVB copolymers
Authors: Guidotti Ronald A. , Even William R.
Published: April 2011

The effects on electrochemical performance of the nitrogen content of disordered carbons derived from polymethacryonitrile (PMAN)-divinylbenzene (DVB) copolymers were examined in galvanostatic cycling tests between 2 V and 0.01 V vs. Li/Li+ in IM LiPF6/ethylene carbonate (EC)-dimethyl carbonate (DMC). The first-cycle reversible capacities and coulombic efficiencies increased with increase in the level of nitrogen for samples prepared at 7000°C. However, the degree of fade also increased. Similar tests were performed on materials that were additionally heated at 1,000° and 1,300°C for five hours. Loss of nitrogen, oxygen, and hydrogen occurred under these conditions, with none remaining at the highest temperature in all cases but one. The pyrolysis temperature dominated the electrochemical performance for these samples, with lower reversible and irreversible capacities for the first intercalation cycle as the pyrolysis temperature was increased. Fade was reduced and coulombic efficiencies also improved with increase in temperate. The large irreversible capacities and high fade of these materials makes them unsuitable for use in Li-ion cells.
The Role of Oxygen in Low-potential Li Insertion in Metal Oxide Anode Materials
Authors: Leroux F. , Nazar L. F.
Published: April 2011

Transition metal oxides are high-capacity lithium storage materials of interest as possible anode materials in the next generation of Li ion batteries. By using X-ray absorption spectroscopy we have obtained an understanding of the process of Li uptake and removal within Na0.25MoO3. Our results show the Li2O matrix on reduction is not inert; Mo-O bonds are reversibly consumed on discharge and are regenerated on charge, with the Li2O matrix acting as the oxygen reservoir. The migration of oxygen atoms from the matrix to the active centers occurs at a voltage below that expected for the Li2O free energy of formation. Polarization on charge is not due only to oxygen migration but also to metal rearrangement within the electrode material. The reversibility of the Mo-O bond formation on repeated cycles is a function of the depth of discharge, with 200 mV being the lower limit.
PEMFC Development at Asahi Glass Co., Ltd.
Authors: Yoshitake M. , Yanagisawa E. , Naganuma T. , Kunisa Y.
Published: April 2011

Perfluorinated ion exchange membranes were studied and the membrane technology for PEMFC has been developed. Thermal stability, mechanical strength, water content, AC specific resistance and gas permeability were measured. The influence of membrane thickness on gas permeability and the influence of incorporation of cations on water content and AC specific resistance of Flemion® and Nafion® 117 were estimated. Gas permeation rates of the membranes decreased in inverse proportion to the increase of the membrane thickness and gas permeability coefficients were nearly constant and independent of the thickness. Hydrogen permeation rates of Flemion®S at 70°C were converted to 2.1 mA/cm2 as cunent density. Flemion®R-electrode assembly showed to maintain stable perfonnance for over 3,500hr. Furthermore, it was found that usage of thinner membranes or one with higher ion-exchange capacity gave not only lower intemal cell voltage but also higher iR-free cell voltage. PTFE-yam embedded type membrane (Flemion®Mc and Sc) and PTFE-flbril dispersed type (Flemion®R12) was examined to afford improvement in mechanical strength at moist and high temperature atmosphere. Flemion®Sc (80!am) was examined to give high cell performance of 0.67V at 0.5A/cm2, 80°C, I ata. Flemior®Mc-electrode assembly was examined to keep stable performance during the life test of over 1,500hr.
Control of A Running H2/02 Fuel Cell With Filled Polymeric Membranes by IMPEDANCE SPECTROSCOPY
Authors: Poinsignon C. , gorrec B Le , Vitter G. , Montella C. , Diard J. P
Published: April 2011

Performances of a H2/O2 polymeric electrolyte fuel cell running at 80°C under 3 bars of gas pressure and using a new filled thermostable ionomeric membrane [sulfonated polysulfone (SPS) filled with phosphatoantimonic acid particles] are analysed with a recent experimental method established for impedance measurements of electrochemical batteries during discharge. A classical impedance-measurement set controls under sinusoidal current perturbation a PEMFC connected in parallel to the load into which it discharges. The impedance diagrams present at high frequencies an inductive behaviour analogous to that observed for batteries, one or two capacitive arcs. The internal resistance of the PEMFC can be estimated. The results demonstrate the possibility to study with classical equipment the impedance of high-capacity and low-impedance FC during their discharge through a constant load.
Improving Gd-Doped Ceria Electrolytes for Low Temperature Solid Oxide Fuel Cells
Authors: Ralph J. M. , Kilner J. A. , Steele B. C. H.
Published: April 2011

Gadolinia-doped ceria electrolyte is being investigated as an alternative electrolyte for solid oxide fuel cells operating at temperatures below 700°C. Measurements were made to determine the effects that small additions of Ca, Pr and Fe to gadolinia-doped ceria have on the bulk and grain boundary conductivities. These small additions (1–2%) of dopant did not alter the bulk conductivity significantly but resulted in a large increase in the grain boundary conductivity. The grain boundary conductivity was similar for all three electrolyte compositions. These results are explained by the possible formation of second phases at the grain boundary, which can incorporate impurity elements. The electronic conductivity in these electrolyte materials was also evaluated, but it was found that the Ca, Pr and Fe additions do not reduce the electronic conductivity compared to a standard Gd-doped ceria sample.
Relationship Between Crystal Structure and Li+-Conductivity in La0.5 Li0.5TiO3 Perovskite
Authors: Alonso J. A. , Ibarra J. , Paris M. A. , Sanz J. , Santamaria J. , LeÓn C. , Vdrez A. , Fermandez M. T.
Published: April 2011

For the first time Li+ cations have been localized within the perovskite structure of the fast Liconductor La0 5Li0.5TiO3 from neutron powder diffraction data. Li cations are fourfold oxygen coordinated, located in the middle of the almost square windows determined by every four TiO6 octahedra, far away from the 12-fold coordinated La vacancy sites that, nevertheless, take part of the path for Li motion across the solid. The large multiplicity of the sites where Li reside (1/6 occupied) accounts for the huge ionic conductivity of this promising Li-conducting material.
Performance of Capacitors Using Organic Electrolytes
Authors: Morimoto T. , Tsushima M. , Che Y.
Published: April 2011

Electric double-layer capacitors(EDLC) based on charge storage at the interface between a high surface area activated carbon electrode and an electrolyte solution are characterized by their long cycle life and high power density in comparison with batteries. However, energy density of electric double-layer capacitors obtained at present is at most 1Wh/kg at a power density of 600W/kg and smaller compared with that of batteries, which limits the applications of the capacitor. Therefore, new capacitors which show larger energy density than that of electric-double layer capacitors are proposed. The new capacitors are hybrid capacitors consisting of activated carbon cathode, Li-doped graphite anode and an organic electrolyte. Maximum voltage applicable to the cell becomes over 4.OV which is larger than that of the electric double-layer capacitor. As a result, discharged energy density of the cell becomes 4Wh/kg at a power density of 600W/kg.
Novel Electrode Materials for Ultracapacitors:Structural and Electrochemical Properties of Sol-Gel-Derived Manganese Dioxide Thin Films
Authors: Pang Suh-Cern , Anderson MarcA
Published: April 2011

Nanoparticulate MnO2thin films fabricated by the sol-gel process have been shown to be an outstanding novel electrode material for Ultracapacitors. The average specific capacitance of sol-gel-derived MnO2thin-films on nickel substrates as determined by cyclic voltammetry ranged from 566 to 698 F/g. These films also exhibited good cycling stability within the potential range of 0.0-0.9V (vs SCE) in unbuffered aqueous electrolyte. Both CV and XPS studies showed that MnO2films have remained chemically and structurally intact after 1,500 cycles. The XRD spectra and SEM micrographs showed that the microstructure of MnO2thin films are highly porous, and poorly crystalline or amorphous in nature. The high specific capacitance of MnO2may be predominantly due to pseudocapacitance associated with homogenous and reversible redox reactions of proton insertion into and out of the MnO2lattice. Any variation in the microstructure and thickness of films might affect proton mobility within the oxide matrix and thereby affecting their cycling behaviors. Further optimization of the cycling behaviors is envisaged with better microstructural and thickness control of these sol-gelderived nanoparticulate MnO2thin films.