Degree Sign Research Articles

Al2O3/ZrO2/Al2O3 High-k Dielectric Stacks on Germanium Substrates Grown by Atomic Layer Deposition at High and Low Temperatures

Al2O3/ZrO2/Al2O3 high-k dielectric stacks have been grown on Germanium substrates by Atomic Layer Deposition (ALD) at temperatures of 150 oC and 200 {degree sign}C from trimethylaluminum (TMA)/water(H2O) and tetrakis-(dimethylamino)zirconium (Zr(NEt2)4)/H2O precursors. The stacks are investigated and compared in terms of Ge-substrate preparation by wet chemical etching (HCl, HF, HI and HBr) and their applicability in "metal oxide semiconductor" (MOS) devices. As a result, we show that in general the deposition of the stacks at lower temperatures (150 oC) leads to lower leakage currents in the range of about one order of magnitude and better interface C-V characteristics. The deposition on GeOx-free substrates obtained by HBr-etching yields to much higher breakdown voltage of the high-k oxide.

Development of Integrated Electronics on Silicon-on-Glass (SiOG) Substrate

Development of integrated electronics on the silicon-on-glass (SiOG) substrate is presented. The SiOG material technology, under development by Corning Incorporated, consists of anodic bonding and an implant-induced separation to transfer a single crystalline silicon film onto a glass substrate. The silicon-glass interface region is characterized by an ultra-strong and thermally stable bond, and includes an in situ barrier layer that is free of mobile ions. Key aspects of the low temperature (< 600 {degree sign}C) CMOS device fabrication process are described, with an emphasis on the design tradeoffs involved to maintain process simplicity (i.e. 6-level mask count, and no VT adjustment). The discussion also provides specific details of the device structure and operation, including mechanisms that compromise device performance identified through electrical simulation.

Ballmilling of Carbon Supports to Enhance the Performance of Fe-based Electrocatalysts for Oxygen Reduction in PEM Fuel Cells

The effect of ballmilling three carbon black supports on the ORR activity of Fe-based electrocatalysts was investigated. Catalysts were prepared by acid-washing these ballmilled carbon supports, then impregnating them with iron(II) acetate and pyrolyzing in NH3 at 950{degree sign}C. It was found that ballmilling the carbon support improved the ORR activity for catalysts made with Black Pearls 2000 (Cabot), but decreased it for those made with N650 and left it relatively unchanged for those made with N234 (both low surface area commercial furnace carbon blacks from Sid Richardson Carbon Corporation). For catalysts made with ballmilled Black Pearls 2000, it was found that ORR activity increases as i) degree of disorder increases, ii) the change in micropore surface area due to pyrolysis increases, iii) nitrogen content increases and iv) crystallite size (La) approaches 24 Aå. No compelling trends were found for catalysts made with ballmilled N234 and N650.

Fullerene-Based Proton-Conductive Material for the Electrolyte Membrane and Electrode of a Direct Methanol Fuel Cell

A fullerene-based compound with proton-conductive functional groups was synthesized and characterized for fuel cell applications. The fullerene-based proton conductor was mixed with polyvinylidene difluoride (PVdF) as a binder to form the electrolyte membranes. The film property can be tuned by changing the ratio of the fullerene derivatives. The proton conductivity of the electrolyte membranes with 67 wt % fullerene derivative was higher than 4.5 × 10-3 S cm-1 at 25 {degree sign}C under relative humidity (RH) of 70%. The membrane-electrolyte assemblies with these materials showed high DMFC performance under ambient conditions.

Atomic Layer Deposited Lanthanum-(Zirconate/Aluminate) Based High-K Dielectric Stacks For Future CMOS-Technology

We present an atomic layer deposition (ALD)-process scheme that allows us to deposit ternary oxides such as LaxAlyOz and LaxZryOz at low process temperatures of 250 {degree sign}C. We combine these oxides with ALD-grown Al2O3 to fabricate various high-k dielectric stacks. By means of metal gate / high-k oxide / semiconductor capacitors we evaluate these materials regarding their electrical properties and thermodynamic stability on silicon and germanium substrates. Concerning the silicon substrates, we observe that superior electrical characteristics are achieved if a thin layer of LaxZryOz is sandwiched between two about 1 nm thick Al2O3 layers. By this approach, hydroxylation of the La-containing oxides is suppressed, minimizing leakage currents as well as oxide- and interface charges.

HfO2 Atomic Layer Deposition Using HfCl4/H2O: The First Reaction Cycle

The growth behavior and film quality of HfO2 deposited by Atomic Layer Deposition (ALD) using HfCl4/H2O depends on the hydroxylation of the exposed surface. In this work, we investigate the dependence of the first HfCl4 chemisorption reaction at 300{degree sign}C on the OH density of the silicon surface. We observe that the hydroxyl density, and hence the Hf deposition, on O3/H2O wet oxides depends on the initial preparation as well as on the stabilization time in the ALD reactor. A good understanding of the initial nucleation behavior is necessary since wet oxides are used in CMOS transistors as surface pre-treatment for aggressively scaled HfO2 gate dielectrics. Moreover, the HfCl4 chemisorption reaction is used to estimate the hydroxylated fraction of these surfaces by means of theoretical models. Finally, the temperature dependence of the OH density, as available in literature, is applied to gain insight in the stoichiometry of the HfCl4 chemisorption reaction.

Low EW poly [perfluorosulfonic acids] /Phosphotungstic acid /Teflon® Supported Membrane Electrode Assemblies

Two different fabrication procedures of catalyst coated membranes (CCMs) using 750 EW poly [perfluorosulfonic acid] ionomer composite membranes that contain phosphotungstic acid are discussed in this study. Cell polarization was obtained at three operating conditions: 80/100/75, 100/70/70, and 120/35/35 (cell temperature {degree sign}C/anode %RH/cathode %RH), and ambient pressure. Hydrogen/air cell performances at 120/35/35 and 400 mA/cm2 varied with the procedure from 0.39V to 0.64 V. Utilization of different fabrication procedures has a significant effect on electrode structure and thus relative overpotential, but little influence on membrane resistance and related ohmic loss. Although much higher performance occurred with the non-heat treatment process, durability issues require heat treatment for stabilization. Further study on the physical and chemical properties of the fabrication process of 750EW poly [perfluorosulfonic acid] ionomers/phosphotungstic acid CCMs is necessary to optimize both the performance and durability of elevated temperature and low relative humidity CCMs.

A Non-Precious Electrocatalyst for Oxygen Reduction Based on Simple Heat-Treated Precursors

An active, non-precious catalyst for electrochemical oxygen reduction was made by heat-treating simple precursors. The current density obtained in an H2/O2 PEM fuel cell was as high as 0.34 A/cm2MEA at 0.65 V. The oxygen-reduction activities of catalysts prepared at different temperatures over the range 800-1050{degree sign}C were shown to be equivalent by the Rotating Ring Disk Electrode (RRDE) method. The nitrogen content, however, was observed by XPS to be highly variable with preparation temperature. Thus, in contrast to some reports in the literature, the surface nitrogen content was not strongly correlated to performance. The nature of the active site(s) is currently under investigation.

Electrical and Chemical Properties of the HfO2/SiO2/Si Stack: Impact of HfO2 Thickness and Thermal Budget

In this paper, we investigate the impact of thermal budget and HfO2 thickness on the chemical and electronic properties of the HfO2/SiO2/Si stack. In the first part, we have seen that high temperature anneal at 750{degree sign}C induces both the regrowth and the reoxidation of the SiO2 interfacial layer. O1s and C1s core level shifts indicate a bias drop along the whole stack of 1.1 eV. This bias drop is ascribed both to the interfacial dipole and to fixed charges in the dielectric. In the second part, ellipsometry and ultraviolet photoelectron spectroscopy are combined to deduce the HfO2 electron affinity (1.8{plus minus}0.2 eV). This value does not change with increasing thermal budget or dielectric thickness.

Epitaxial Growth of Ge Thick Layers on Nominal and 6{degree sign}C off Si(001); Ge Surface Passivation by Si

not Available.

Growth of ZnO Epitaxial Films and Structures in Water at 90{degree sign}C

not Available.

Post-Silicidation Annealing Effects on Electrical and Structural Properties of NiPt Germanosilicide

In this work NiPt germanosilicides were fabricated with 20 nm thick Ni and different Pt layer (3, 6, 9, and 12 nm thick) by rapid thermal annealing at 300 {degree sign}C, and then the post-annealing effects on these samples were investigated in the temperature range between 400 ºC to 750 ºC. The obtained silicide samples were analyzed using several characterization techniques. Smooth and uniform Ni(Pt) monogermanosilicide films have been observed for all analyzed samples. For annealing temperatures lower than 500 ºC the samples exhibited thermal stability with sheet resistances of 10 - 30 Ω/sq, and sheet resistance degradation for annealing temperatures of 500 {degree sign}C and above. Structural transformation of formed silicide to Ge rich Ni(Pt)Si1-uGeu films was found to occurs with increasing of the annealing temperature. Surface analysis revealed morphological instability of germanosilicides and strong tendency for agglomeration and mounds-like structure formation, that leads to an abrupt increase in the sheet resistance.

Characteristics of Titanium Oxide Gate Nmosfet Formed by E-Beam Evaporation with Additional Rapid Thermal Oxidation and Annealing

High k insulators, such as titanium oxide, have been obtained by Ti e-beam evaporation, using different thicknesses of 5nm, 10nm and 20nm with additional rapid thermal oxidation and annealing at temperature of 960{degree sign}C for 40s. Characterization by Fourier transform infrared analyses reveals the Ti-O and Si-O bonds, which confirms titanium oxide formation. Raman spectroscopy identified that these films present rutile and anatase phases. nMOSFETs and MOS capacitors, with Al electrodes and final sintering at 450{degree sign}C for 10min in forming gas, were fabricated. MOS capacitors were used to obtain capacitance-voltage measurements, and to extract the Equivalent Oxide Thickness from the films, resulting in values between 16.3nm and 19.7nm, and effective charge densities of about 1011cm-2. nMOSFET electrical characteristics, such as threshold voltages between 0.39V and 0.43V and sub-threshold slopes between 61mV/dec and 100mV/dec, were obtained. These results indicated that the TiOx films are suitable gate insulators for MOS devices.

Time-of-Flight Flow Sensor Microstructure Using Free-Standing Polysilicon Filaments

In this work we present a time-of-flight flow microsensor implemented using surface micromachining in combination with sacrificial layer technology. This approach allows obtaining free-standing polysilicon microfilaments that can be used as heaters or temperature sensors. The obtained microstructure can find applications in many important areas as process and environmental control, biomedicine and instrumentation. Numerical simulation showed a compromise between the size of the device (distance heater to sensor) and the range of volumetric flow to be detected. Also, the proposed structure is suitable for the detection of low volumetric flows and presents a response time of the order of milliseconds. Experimental results demonstrate that the heater can be operated at a temperature of the order of 100{degree sign}C with a power dissipation of ~ 30 mW. Also, tests with the implemented flow sensor structure and characterization apparatus revealed a good agreement with respect to the numerical simulation, confirming the predicted behavior.

DC Performance and Low Frequency Noise in n-MOSFETs using Self-Aligned Poly-Si/SiGe Gate

The characterization of an n-MOS transistor with poly-Si/SiGe Gate fabricated with the CMOS process entirely developed in the Center for Semiconductor Components (CCS) at UNICAMP is presented. The Gate layer was grown by vertical LPCVD at 800 {degree sign}C. The resultant transistor has a channel region with oxide thickness of 30 nm and self-aligned thick S/D region. The DC and Gm characteristics of poly-Si/SiGe n-MOS transistor are reported. The turn-on in the I-V characteristics increases and at a drain-to-source bias VDS of +0.1 V nMOSFETs with 3 µm gate length had peak transconductance (µS) increased as well, compared with conventional n-MOS with poly-Si gate. The Gm characteristics and low frequency noise 1/f of the n-MOS transistors are studied using devices sizes with width of 20 µm and several lengths. Promising devices for RF and microwave circuit applications, show low 1/f and high values of transconductance.

Phase Behavior and Conductivity of Et4NTFSI-LiTFSI Mixtures - A Model System for Ionic Liquid Lithium Battery Electrolytes

The phase behavior and ionic conductivity of tetraethylammonium bis(trifluoromethanesulfonyl)imide (Et4NTFSI) salt mixtures with LiTFSI have been examined. Two new (1-x) Et4NTFSI-(x) LiTFSI (x = 0.50 and 0.67) mixed salt crystalline phases form. These mixtures serve as models for the previously studied N-alkyl-N-methylpyrrolidinium salt (1-x) PYR1RTFSI-(x) LiTFSI mixtures. New interesting variations in the ionic conductivity are observed which are attributed to the solid-solid phase transitions of the neat Et4NTFSI salt creating disordered plastic crystalline phases. Although Et4NTFSI melts at 102{degree sign}C and LiTFSI melts at 236{degree sign}C, the two salts form a eutectic with a melting temperature of 31-33{degree sign}C.

Solvent-free, PYR1ATFSI Ionic Liquids-based Ternary Polymer Electrolyte Systems. II. Battery Tests

In this manuscript are reported the battery tests of ternary PEO electrolytes containing N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquid. The results of the tests show that lithium-metal, solvent-free batteries based on these ternary electrolytes are able to deliver the full capacity in low-rate discharge C/50 rate at room temperature. Optimum performance, however, is obtained at slightly higher temperatures (40{degree sign}C). The performance of ionic liquid-containing polymer batteries at 40{degree sign}C are comparable to analogous ionic liquid-free Li/LiFePO4 systems at 90{degree sign}C. The P(EO)10LiTFSI + 0.96 PYR1n4TFSI batteries showed long cycle life and high coulombic efficiency.

Study of the Electrochemical Step of Novel Active Metal Alloy Thermochemical Cycles for Hydrogen Production

Alternative thermochemical cycles are promising technologies for producing hydrogen by taking advantage of the waste heat from nuclear reactors. These cycles, which consist of two or more interrelated chemical reactions and are typically realized at moderately elevated temperatures (500-750 {degree sign}C), are still at early stages of research, and more information is needed to evaluate their potential. The first proof-of-principle studies were performed for the electrolysis reaction of the K-Bi thermochemical cycle. Experiments were carried out in a ceramic reactor at 580 {degree sign}C, and the experimentally measured electrolysis potentials were found to be close to the theoretically estimated values. This suggests that the voltage efficiency of the reaction can be as high as 90%.

Electrochemical Performance of Nanostructured LiMn1.5Ni0.5O4 Spinel at Elevated Temperature

In order to identify the failure mechanism of LiMn1.5Ni0.5O4 (LMN) spinel at elevated temperature (60{degree sign}C), the effect of carbon black, Mn3+ content and charge/discharge state storage conditions were studied. It was identified that the main degradation mechanism at elevated temperature was the systematic impedance rise rather than intrinsic capacity loss. At this early stage we attribute most of this to the anodic decomposition of the electrolyte on the surface of the spinel. We introduce a surface conversion process to improve the elevated temperature performance of the spinel. The surface converted materials exhibited much lower impedance rise with improved capacity retention and rate capability at elevated temperature.

Thin Film Electrochemical Deposition at Temperatures up to 180 {degree sign}C for Photovoltaic Applications

This paper presents several examples of thin films synthesized by electrochemical deposition in view of their application in photovoltaic. An overview of the deposition of CdTe, d-SnS, Bi2S3, ZnO and CuInSe2 is given to illustrate the versatility of the electrodeposition method. CdTe/CdS and CuInSe2/CdS solar cells made by electrochemical methods are in the stage of pilot lines, while the research on ZnO/CdSe/CuSCN solar cell based on an advanced concept of sensitization of wide bandgap materials is briefly described.