There is continued interest in developing more stable contacts to a variety of GaN-based devices. In this paper we give two examples of devices that show improved thermal stability when boride, nitride or Ir diffusion barriers are employed in Ohmic contact stacks. AlGaN/GaN High Electron Mobility Transistors (HEMTs) were fabricated with Ti/Al/X /Ti/Au source/drain Ohmic (where X is TiB2, ZrN, TiN, TaN or Ir) contacts and subjected to long-term annealing at 350°C. For GaN layers with an electron concentration of ∼3×1017 cm-3, the minimum specific contact resistance achieved is 6×10-5 Ω cm2 for Ti/Al/TiN/Ti/Au after annealing at 800°C. The specific contact resistance was found to strongly depend on the doping level, suggesting that tunneling is the dominant mechanism of current flow. By comparison with companion devices with conventional Ti/Al/Ni/Au Ohmic contacts, the HEMTs with boride-based Ohmic metal showed superior stability of both source-drain current and transconductance after 25 days aging at 350°C. The gate current for standard HEMTs increases during aging and the standard Ohmic contacts eventually fail by shorting to the gate contact. Similarly, InGaN/GaN multiple quantum well light-emitting diodes (MQW-LEDs) were fabricated with either Ni/Au/TiB2/Ti/Au or Ni/Au/Ir/Au p-Ohmic contacts. Both of these contacts showed superior long-term thermal stability compared to LEDs with conventional Ni/Au contacts.

In the last decade, GaN-on-Si has progressed from fundamental crystal growth studies to product realization and reliability demonstration. GaN-on-Si HEMTs addressing cellular, WiMAX, and broadband RF applications are now commercially available and offer GaN performance attributes in a cost-competitive platform. This presentation will briefly describe the underlying GaN-on-Si material, process, and packaging technology, then focus primarily on performance of these products in both commercial and military applications.

All Nitronex NRF1 GaN-on-Si products are grown by MOCVD on 100 mm float-zone Si (111) substrates. A proprietary, strain-compensating (Al,Ga)N transition layer and an amorphous SixAl1-xNy nucleation layer are employed to accommodate lattice and thermal expansion mismatch between the substrate and the epilayers. The wafer fabrication process employs Ti/Al-based ohmic contacts, ion implant device isolation, 0.5 um dielectrically-defined gates, gold airbridge interconnects, and through-wafer source vias. Typical inline DC parametrics include 2DEG sheet resistance of 490 ohms/sq., on-resistance of 3 ohm-mm, peak drain current density of 830 mA/mm, and breakdown voltage of >100V. Packaging solutions include traditional LDMOS-style air cavity outlines with thermally-enhanced flange materials and low-cost plastic SOIC.

A family of devices addressing emerging OFDM-based applications such as WiMAX has been developed. WiMAX amplifiers require several watts of linear output power with frequency band allocations ranging from 2.3 to 5.8 GHz and instantaneous bandwidth up to ∼15%. Translated to the transistor level, this implies simultaneous high frequency and high voltage capability – attributes well-suited to the inherent advantages of GaN-based devices. The flagship product in this family is NPT25100, delivering 125W of peak envelope power at 2.5 GHz. Under 2.5 GHz single-carrier OFDM modulation and 10 MHz channel bandwidth, this device produces 10W linear power at 2.0% EVM with 16.5dB associated gain and 26% drain efficiency. The excellent bandwidth of NRF1 devices enables the same device to operate at cellular frequencies from 2.11 - 2.17 GHz, producing >20W average power at an adjacent channel power ratio of -35 dBc.

Primary military insertion opportunities include communications (e.g., JTRS - Joint Tactical Radio System) and electronic warfare (e.g., jammers). For EW applications, broadband operation reduces system-level component count and decreases weight / footprint. A family of 48V GaN-on-Si broadband HEMTs has been developed to deliver power levels from 40W - 180W in a compact package. In the highest power case, packaged “power density” (defined as peak output power divided by package volume) reaches ∼650 W/cm3. These power levels – in an outline suitable for highly portable systems – enable improved communications transmit distance and extend the umbrella size of electronic protection units.

We report the influence of surface treatment, annealing temperature and metal bilayer thickness on the specific contact resistance (ρc) of Au/Ni ohmic contacts to p-GaN and p-AlGaN. Ohmic contact on p-GaN with a hole concentration of 6.5 x 1017 cm-3, shows the lowest ρc of ˜9.2 x 10-6 Ω cm2, when GaN was treated in HCl:H2O (3:1) solution before metal deposition and annealed at 500°C for 10 minutes in 90% N2 and 0% O2 atmosphere. Similar procedure applied on p-AlxGa1-xN (x = 5-7%), with a hole concentration of 2.3 x 1017 cm-3, yields a ρc of 1.8 x 10-4 Ω cm2. An increase is observed in ρc when Mg doping exceeds 4 x 1019 cm-3 in both p-GaN and p-AlGaN. This is attributed to Mg self compensation. This increase is more pronounced in AlGaN which we attribute to the presence of residual native aluminum oxides.

Electrically Conductive Polymer nanocomposites have attracted lots of attention in the last years, especially for their sensitivities to external solicitations, like temperature or pressure variation. This work concerns the modelling of toluene diffusion behaviour in poly(ethylene-co-ethyl acrylate) (EEA)-carbon nanoparticles (CNP) CPC. One of the main objective of our work was to control and model the physical mechanisms involved in this type of material during sorption and desorption phases in the presence of solvent vapour. Two approaches was explore, thin layer to study quick electrical response and thick layer to look after swelling effect induces by toluene. The thick layer mass measurement was compare with our diffusion model.

SiC films were deposited on Si substrate by low pressure hot-wall CVD using C3H8 (5% in H2)-SiH4 (2.5% in H2)-H2 gas system at 1270°C and 1370°C. In this paper, we compare the electrical characteristics of MOS capacitors fabricated on the 3C-SiC films grown at high and low temperatures, 1370°C and 1270°C, respectively. Although the cross-sectional TEM images indicate similar micro-structural quality of the SiC/Si interface for both temperatures, a quicker elimination rate of stacking faults with increasing thickness was observed in the films grown at 1370°C. Rocking curves from XRD measurements also indicate better crystallinity of the films grown at 1370°C. On the other hand, atomic force microscopy shows that the average surface roughness reduces with the reduction in growth temperature. MOS capacitors were made on films grown at both temperatures and characterized by high-frequency capacitance-voltage (HFCV), conductance-voltage (G-V), and current-voltage (I-V) measurements at room temperature. The MOS capacitors fabricated on both films exhibit good and almost identical C-V characteristics. Measurements of current-voltage characteristics in accumulation region showed smaller leakage for the film deposited at 1270°C. It is concluded that the decrease of the deposition temperature from 1370°C to 1270°C does not bring any remarkable negative impact on the interface properties of fabricated MOS capacitors.

A two-phase intermetallic alloy composed of Ni3Al (L12) and Ni3V (D022) was plasma-nitrided or -carburized in dependence of temperature and time. It was found that the hardness of the surface layer of the present intermetallic alloy was enhanced by both plasma-nitriding (PN) and -carburizing (PC), and primarily depended on treating temperature; the maximum surface hardness of the alloy was shown by nitriding at around 850 K and by carburizing at 1025 K. In addition, the hardened layers due to PN and PC effectively kept their hardness up to a high temperature. The XRD analysis revealed that vanadium nitride (VN) and vanadium carbide (VC) were formed in the surface of the nitrided and carburized alloy, respectively, suggesting that the enhanced surface hardness was attributed to the dispersion hardening due to the nitrides and carbides.

Tight control of defects is pivotal for semiconductor technology. However, even the basic defects are not entirely understood in silicon carbide. In the recent years significant advances have been reached in identification of defects by combining the experimental tools like electron paramagnetic resonance and photoluminescence with ab initio calculations. We summarize these results and their consequences in silicon carbide based technology. We show recent methodological developments making possible the accurate calculation of absorption and emission signals of defects.

Garnets, A3B2C3O12, are considered to be potential host phases for the immobilization of high-level nuclear waste as they can accommodate a number of elements of interest, including Zr, Ti and Fe. The naturally occurring garnet, kimzeyite, Ca3(Zr,Ti)2(Si,Al,Fe)3O12, can contain ˜30wt% Zr. An understanding of the radiation tolerance of these materials is crucial to their potential use in nuclear waste immobilization. In this study two synthetic analogues of kimzeyite of composition Ca3Zr2FeAlSiO12 and Ca3Hf2FeAlSiO12 were monitored in situ during irradiation with 1.0 MeV Kr ions using the intermediate voltage electron microscope-Tandem User Facility (IVEM) at Argonne National Laboratory. The structure of these materials was previously determined by neutron diffraction and 57Fe Mössbauer spectroscopy. Ca3Zr2FeAlSiO12 and Ca3Hf2FeAlSiO12 have very similar structural properties with cubic Ia3d symmetry, the only significant difference being the presence of Zr and Hf, respectively, on the 6 coordinated B sites.

This work describes the effect of addition of O2 and NH3 to the N2 ambient used during the UV cure process of low-k materials. The effects of O2 give an acceleration of the cure process, resulting in increased film modulus and shrinkage. The use of NH3 resulted in a retardation of UV cure, explained by the higher absorption cross section of NH3 at the wavelengths used.

Monte-Carlo simulations calculate the photon collection of fluorescent collectors in photovoltaic systems. We focus on two collector geometries: solar cells mounted at the collector sides, and solar cells covering the back of the collector. A mirror covers the bare back sides of both systems. On top lies optionally a photonic structure, which acts as an energy selective filter. Ideal systems in their radiative limits are compared to systems where loss mechanisms in the dye, at the mirror, or the filter are included. The examination of loss mechanisms in photovoltaic systems with fluorescent collectors enables us to estimate quality limitations of the used materials and components.

To impart machinability to hard and brittle AlN ceramics without losing their high thermal conductivity, a homogeneous dispersion of fine BN particles in an AlN matrix was investigated. A homogeneous dispersion of BN particles was obtained by pressureless sintering of turbostatic BN-coated AlN nanocomposite powder (AlN–BN nanocomposite powder), which was prepared by reducing and heating AlN particles containing a mixture of boric acid, urea, and carbon. Though AlN is slightly oxidized by boric acid during the reduction, the addition of carbon reduced the oxygen content of the AlN–BN composite powder by carbothermal reduction of the oxidized AlN particles. As a result, the thermal conductivity of the sintered material increased with decreasing oxygen content of the nanocomposite powder. AlN–BN nanocomposites containing more than 20 vol% BN showed high strength, machinability, and relatively high thermal conductivity in comparison with the conventional microcomposites.

The interaction of amorphous silicon and aluminum films to achieve polycrystalline silicon has been investigated using transmission electron microscope equipped with in-situ heating holder. Carbon coated nickel grids were used for TEM studies. An ultra high vacuum cluster tool was used for the deposition of a ∼50nm a-Si films and a vacuum deposition system was used to deposit a ∼50nm Al films on a-Si film. The microstructural features and electron diffraction in the plain view mode were observed with increase in temperature starting from room temperature to 275 °C. The specimen was loaded inside TEM heating holder. The temperature was measured and kept constant for 5 minutes during which the microstructure at fixed magnification of X63K was recorded and the electron diffraction pattern of the same area was also recorded. The temperature was then increase and fixed at desired value and microstructure and EDP were again recorded. The temperatures used in this experiment were 30, 100, 150, 200, 225, 275°C. A sequential change in microstructural features and electron diffraction pattern due to interfacial diffusion of boundary between Al and amorphous Si was investigated. Evolution of polycrystalline silicon with randomly oriented grains as a result of a-Si and Al interaction was revealed. After the in-situ heating experiment the specimen was subjected to high resolution TEM and EDS investigations after removing the excess Al. The EDS analysis of the crystallized specimen was performed to locate the Al distribution in the crystallized silicon. These studies show that the Al induced crystallization process can be used to prepare polycrystalline as well as nanocrystalline silicon by controlling the in-situ annealing parameters. The investigations are very useful as the nanocrystalline silicon is being investigated for its use in developing high efficiency silicon solar structures.

The thermodynamics and kinetics of hydrogen dissolved in structural metals is often not addressed when assessing phenomena associated with hydrogen-assisted fracture. Understanding the behavior of hydrogen atoms in a metal lattice, however, is important for interpreting materials properties measured in hydrogen environments, and for designing structurally efficient components with extended lifecycles. The assessment of equilibrium hydrogen contents and hydrogen transport in steels is motivated by questions raised in the safety, codes and standards community about mixtures of gases containing hydrogen as well as the effects of stress and hydrogen trapping on the transport of hydrogen in metals. More broadly, these questions are important for enabling a comprehensive understanding of hydrogen-assisted fracture. We start by providing a framework for understanding the thermodynamics of pure gaseous hydrogen and then we extend this to treat mixtures of gases containing hydrogen. An understanding of the thermodynamics of gas mixtures is necessary for analyzing concepts for transitioning to a hydrogen-based economy that incorporate the addition of gaseous hydrogen to existing energy carrier systems such as natural gas distribution. We show that, at equilibrium, a mixture of gases containing hydrogen will increase the fugacity of the hydrogen gas, but that this increase is small for practical systems and will generally be insufficient to substantially impact hydrogen-assisted fracture. Further, the effects of stress and hydrogen trapping on the transport of atomic hydrogen in metals are considered. Tensile stress increases the amount of hydrogen dissolved in a metal and slightly increases hydrogen diffusivity. In some materials, hydrogen trapping has very little impact on hydrogen content and transport, while other materials show orders of magnitude increases of hydrogen content and reductions of hydrogen diffusivity.

In this work we present the study of fabrication, Ge incorporation, structure and electronic properties of nano-structured GeySi1-y:H films with y>0.5 prepared by low frequency (LF) PECVD. GeySi1-y:H films were deposited by LF PECVD at a frequency f= 110 kHz from SiH4+GeH4+H2 gas mixture. SiH4 and GeH4 flows were varied to fabricate the films in wide range of 0<y<l. Hydrogen dilution was varied in the range of RH =20 to 80. Structure of the films was studied by AFM and SEM with consequent image processing to extract statistical parameters such as grain distribution and mean values. Composition of the films was characterized by SIMS and EDX. Electronic properties were characterized by temperature dependence of conductivity, spectral dependence of optical absorption. Sub-gap absorption was characterized by Urbach energy, EU; and defect absorption, αD. We observed grain like nano-structure with Gauss distribution of grain diameters by both AFM and SEM measurements. The most interesting films had mean grain diameter<D> = 24.0±0.7 nm, dispersion D=11.0±0.2 nm and fill factor FF=0.313, Ge content y=0.96-0.97(by SIMS and EDS). These films showed also the lowest values of Urbach energy EU = 0.030 eV and low defect absorption αD = 5×102 cm −1 (at photon energy hv = 1.04 eV) indicating on low density of localized states in mobility gap. Doped films have been also fabricated and studied. Finally we shall discuss application of the above films in photovoltaic devices.

We have synthesized polyester systems containing pendant iptycene units and compared their mechanical/structural properties to a homologous reference polymer wherein benzene replaces iptycene units. Iptycenes have unique structural properties called internal molecular free volume (IMFV). The incorporation of iptycene into polyester backbones results in a polymer chain contour resembling “molecular barbed wire.” The contribution of iptycene to the mechanical properties of polyesters is significant and robust across concentration and processing conditions. The triptycene polyester films displayed a nearly 3-fold increase in Young's modulus, an approximately 3-fold increase in strength, and a more than 20-fold increase in strain to failure. We proposed that the presence of triptycene introduces two mechanisms for the enhancement of tensile mechanical properties: molecular threading and molecular interlocking.

Nanocrystals embedded in an oxide matrix have been fabricated by annealing SiGeO films deposited by LPCVD. The composition of the oxide layers and its evolution after annealing as well as the presence and nature of nanocrystals in the films have been studied by several experimental techniques. The results are analyzed and discussed in terms of the main deposition parameters and the annealing temperature.

Solution electrospinning was used to prepare fibers of both the stable – trigonal α and “metastable” – orthorhombic β complexes between poly(ethylene oxide) (PEO) and urea. The 300-800 nm fibers were highly crystalline and both types presented a relatively large level of molecular orientation. Characterization of the poorly-studied β complex was performed using wide-angle X-ray diffraction, infrared spectroscopy, optical microscopy and differential scanning calorimetry. It was shown that β complex possesses a 3:2 PEO:urea stoichiometry, in contrast with a previously suggested 1:1 molar ratio, and that the α inclusion complex keeps the 4:9 molar ratio as when prepared by the conventional co-crystallization method. A new structural model was suggested for the β complex, in which the unit cell would contain 12 PEO repeat units (4 chains in the ab plane with 3 repeat units along the c axis) and 8 urea molecules arranged in a ribbon-like structure and intercalated between the two PEO layers. This layer-structured β complex is quite different from the usual channel-like α inclusion complex and results in different phase transitional properties.

Evaluation of the transport and retardation properties of rock matrices that serve as host rock for nuclear waste repositories necessitates their thorough pore-space characterization. Relevant properties to be quantified include the diffusion depth and volume adjacent to water conducting features. The bulk values of these quantities are not sufficient due to the heterogeneity of mineral structure on the scale of the expected transport/interaction distances. In this work the 3D pore structure of altered granite samples with porosities of 5 to 15%, taken next to water conducting fractures at 180 200 m depth in Sievi, Finland, was studied. Characterization of diffusion pathways and porosity were based on quantitative autoradiography of rock sections impregnated with C14-labelled polymethylmethacrylate (PMMA). Construction of 3D structure from PMMA autoradiographs was tested. The PMMA method was augmented by field emission scanning electron microscopy and energy-dispersive X-ray analyses (FESEM/EDAX) in order to study small pore-aperture regions in more detail and to identify the corresponding minerals. The 3D distribution of minerals and their abundances were determined by X-ray microtomography. Combining the mineral specific porosity found by the PMMA method with these distributions provided us with a 3D porosity distribution in the rock matrix.

Dental remineralization may be achieved by mediating the interactions between tooth surfaces with free ions and biomimetic peptides. We recently developed octuplet repeats of aspartate-serine-serine (DSS-8) peptide, which occurs in high abundance in naturally occurring proteins that are critical for tooth remineralization. In this paper, we evaluated the possible role of DSS-8 in enamel remineralization. Human enamel specimens were demineralized, exposed briefly to DSS-8 solution, and then exposed to concentrated ionic solutions that favor remineralization. Enamel nano-mechanical behaviors, hardness and elastic modulus, at various stages of treatment were determined by nanoindentation. The phase, microstructure and morphology of the resultant surfaces were characterized using the grazing incidence X-ray diffraction (GIXD), variable pressure scanning electron microscopy (VPSEM), and atomic force microscopy (AFM), respectively. Nanoindentation results show that the DSS-8 remineralization effectively improves the mechanical and elastic properties for demineralized enamel.

We report the energy band alignment of Ge2Sb2Te5 and a variety of common complementary-metal-oxide-semiconductor (CMOS) compatible materials. These materials include silicon, silicon oxide, hafnium oxide, silicon nitride as well as nickel silicide. High-resolution X-ray photoelectron spectroscopy was employed as the main tool to obtain the core-level spectra, the valence band spectra, and the energy loss spectra. A precise determination of the valence band offsets of Ge2Sb2Te5 and the various materials were obtained. The conduction band offsets were then determined. The energy band line-ups of Ge2Sb2Te5 and these CMOS compatible materials were established.