High Germanium Content Research Articles

Rapid characterization of the ultraviolet induced fiber Bragg grating complex coupling coefficient as a function of irradiance and exposure time

We report the application of optical frequency domain reflectometry and a discrete-layer-peeling inverse scattering algorithm to the spatial characterization of the UV induced complex coupling coefficient during fiber Bragg grating growth. The fiber grating is rapidly characterized using this technique to give irradiance dependent growth as a function of exposure time, thereby providing the complete characterization of the coupling coefficient in the form of a "growth surface," which is related to the fiber's photosensitivity. We compare measurements of fiber Bragg grating growth in SMF-28 when exposed to continuous wave 244 nm irradiation from 0 to 90 W cm(-2) for exposure times up to 3230 s with a selection of other fibers including high germanium concentration fiber and erbium doped fiber.

Microstructured fibers with highly nonlinear materials

We investigated the effect of modification of silica with high concentrations of germanium up to 36 mol% as well as with highly polarizable dopants (e.g., barium and lanthanum) on optical behavior of microstructured fibers. The goal was to investigate the influence of doping on several properties like fiber attenuation, supercontinuum generation and birefringence in microstructured optical fibers (MOF).

Si/SiGe near-infrared photodetectors grown using low pressure chemical vapour deposition

Near-infrared photodetectors have been fabricated using standard CMOS processes in conjunction with the multilayer growth of Si/SiGe0.06 using low-pressure chemical vapor deposition (LPCVD). Cross-section scanning electron microscopy (SEM) indicates the existence of quantum dot like corrugations in devices with particularly thick SiGe0.06 quantum wells. With an accumulation of germanium atoms at the crest of such features and commensurate high germanium concentration we see a considerable enhancement of the long wavelength detection sensitivity of photodetectors in the range 1100–1300 nm. By fitting experimental data the minimum energy gap of the structure is found to be 0.88 eV corresponding to a germanium concentration of around 15%.

Fabrication of SOI MOSFET by Separation by Bonding Silicon Islands (SBSI) Method

Local SOI MOSFETs were fabricated in the designed area on a Si bulk substrate using the separation by bonding silicon islands (SBSI) method. We describe the key fabrication process technology and show the electrical characteristics. The SBSI method consists of three key technologies: epitaxial growth of SiGe having high germanium concentration, lateral selective etching of SiGe, and bonding of buried oxides (BOX) grown in the gap. Epitaxial growth of 30 nm-thick Si0.63Ge0.37 without crystalline defects was achieved by reducing the growth temperature to as low as 450 ºC. The Si/Si0.63Ge0.37 structure allowed lateral etching of Si0.63Ge0.37 with a length of 1 mm in 2 minutes and with negligible etching of Si in a solution of hydrofluoric acid and nitric acid. Fabricated MOSFETs showed excellent characteristics, which were comparable to those of SOI wafers, although a small gap still remained in the BOX.

Assessing the impact of leather industries on the quality of water discharged into the East China Sea from Wenzhou Watersheds

Ammonium nitrogen and total germanium are among the main pollutants in the wastewater discharged from the leather industry. The intake of high concentrations of ammonium nitrogen and/or total germanium harms human health and biological species, as is well documented in literature. This paper focuses on assessing the trends of ammonium nitrogen and total germanium concentrations through time in two watersheds (Aojiang and Oujiang) in the Wenzhou metropolitan area of Zhejiang Province and their relationships with the released wastewater using regression and correlation statistics. The paper also utilizes the integrated pollution index to evaluate water quality in the two watersheds. Preliminary results show that, from 1992 to 1998 in the Aojing watershed, the concentrations of ammonium nitrogen and total germanium increased 13 and 14 times, respectively, decreasing somewhat after 1998, while between 1992 and 1997 in the Oujiang watershed, the concentrations increased, then decreased after 1997. The concentrations of ammonium nitrogen and total germanium are positively related to the amount of released wastewater. The concentrations of ammonium nitrogen and total germanium exceeded water standards 12 and 3 times, respectively, in Pingyang county of the Aojiang watershed, 14 and 3.3 times in Lucheng District of the Oujiang watershed, and 14 and 3.8 times in the Ouhai Oujiang watershed, respectively. In Pingyang county of the Aojiang watershed, the water quality degraded from Type III in 1992 to over Type V in 2003, and in the Oujiang watershed, the water quality degraded from Type II to over Type IV in 1999, when they were compared with the water quality standards. The water quality slightly improved in 2003 for the Oujiang watershed. It appears that pollution did have a direct positive correlation with leather industry production in the Pingyang Aojing watershed, while there was a negative correlation between the two in the Oujiang watershed. In these two watersheds, the integrated pollution index did not appear to relate to population dynamics and agricultural production. This paper also discusses the current new methodologies and approaches adopted nationally and internationally to reduce the contaminants and purify the environment for maintaining a sustainable and healthier environment in Wenzhou.

High-mobility low band-to-band-tunneling strained-Germanium double-gate heterostructure FETs: Simulations

Large band-to-band tunneling (BTBT) leakage currents can ultimately limit the scalability of high-mobility (small-bandgap) materials. This paper presents a novel heterostructure double-gate FET (DGFET) that can significantly reduce BTBT leakage currents while retaining its high mobility, making it suitable for scaling into the sub-20-nm regime. In particular, through one-dimensional Poisson-Schrodinger, full-band Monte Carlo, and detailed BTBT simulations, the tradeoffs between carrier transport, electrostatics, and BTBT leakage in high-mobility sub-20-nm Si-strained SiGe-Si (high germanium concentration) heterostructure PMOS DGFETs are thoroughly analyzed. The results show a dramatic (>100/spl times/) reduction in BTBT and an excellent electrostatic control of the channel while maintaining very high drive currents and switching frequencies in these nanoscale transistors.

High-temperature growth of very high germanium content SiGe virtual substrates

We have first of all studied (in reduced pressure–chemical vapour deposition) the high-temperature growth kinetics of SiGe in the 0–100% Ge concentration range. We have then grown very high Ge content (55–100%) SiGe virtual substrates at 850 °C. We have focused on the impact of the final Ge concentration on the SiGe virtual substrates’ structural properties. Polished Si 0.5Ge 0.5 virtual substrates were used as templates for the growth of the high Ge concentration part of such stacks, in order to minimize the severe surface roughening occurring when ramping up the Ge concentration. The macroscopic degree of strain relaxation increases from 99% up to values close to 104% as the Ge concentration of our SiGe virtual substrates increases from 50% up to 100% (discrepancies in-between the thermal expansion coefficients of Si and SiGe). The surface root mean square roughness increases when the Ge concentration increases, reaching values close to 20 nm for 100% of Ge. Finally, the field (the pile-up) threading dislocations density (TDD) decreases as the Ge concentration increases, from 4×10 5 cm −2 (1–2×10 5 cm −2) for [ Ge ] = 50 % down to slightly more than 1×10 5 cm −2 (a few 10 4 cm −2) for [ Ge ] = 88 % . For [ Ge ] = 100 % , the field TDD is of the order of 3×10 6 cm −2, however.

High Germanium Content Strained SGOI by Oxidation of Amorphous SiGe Film on SOI Substrates

High Germanium Content Strained SGOI by Oxidation of Amorphous SiGe Film on SOI Substrates

Effects of germanium content on sol-gel GeO2/γ-glycidoxypropyltrimethoxysilane hybrid planar waveguides prepared at low temperature

The effects of germanium content on the structural and optical properties of GeO2/γ-glycidoxypropyltri-methoxysilane hybrid materials processed by the sol-gel technique are studied for optical waveguide applications. Glass planar waveguide thin films with thickness of more than 2.0 μm are prepared by a single-coating process and low-temperature heat treatment from this high germanium content hybrid material. Multimode light guiding is demonstrated in the hybrid planar waveguide films. The properties of the planar waveguide films such as refractive index, thickness, and transparency as well as structural characterization using the prism coupling technique, atomic force microscopy, thermal gravimetric analysis, UV-visible spectroscopy, and Fourier transform infrared spectroscopy, are systematically studied. These results indicate that the refractive index increases and film thickness decreases with an increase in GeO2 content, and thus dense, low absorption, and high transparency in the visible range waveguide films can be obtained at a low temperature. Waveguide propagation characteristics, at 632.8 and 1550 nm, are studied. The propagation loss of the hybrid waveguide film with a GeO2 content of 0.3 M is measured to be 1.2 dB/cm at the wavelength of 1550 nm.

Effects of Ge concentration, boron co-doping, and hydrogenation on fiber Bragg grating characteristics

In this paper, we compare photosensitivity, refractive-index modulation depth, spectral bandwidth, and excess loss through the inscription of fiber Bragg gratings in unloaded and hydrogen-loaded fibers with low- or high-germanium concentration and boron co-doping. Gratings in hydrogenated boron co-doped fibers possess the highest reflectivity, but with wider bandwidth and strong excess loss. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 44: 148–152, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20572

Challenges of high Ge content silicon germanium structures

High germanium content silicon germanium (SiGe) structures are of special importance for future silicon based microelectronics. The lattice mismatch of more than 1% causes large elastic strain, the energy of which increases with the square of the strain. This high strain energy does not allow a straight continuation of the growth and science of pseudomorphic SiGe as in the existing very successful heterobipolar transistor technology. This paper overviews three different strategies to overcome the barrier connected with lattice mismatched hard, covalent bound material couples. These strategies utilize at least one of the ingredients: metastable growth, strain adjustment and corrugated surface morphology. Impact on resonance phase operation of transistors, on the high performance symmetrical CMOS and on self-aligned quantum dot formation is shown.

Germania/ormosil hybrid materials derived at low temperature for photonic applications

We report on germania/organically modified silane (ormosil) hybrid materials produced by the sol–gel technique for photonic applications. Acid-catalyzed solutions of γ-glycidoxypropyltrimethoxysilane mixed with germanium isopropoxide have been used as precursors for the hybrid materials. Planar waveguide films with a thickness of about 2 μm have been prepared by a single spin-coating process and low-temperature heat treatment from these high germanium content hybrid materials. Atomic force microscopy, thermal gravimetric analysis, UV–visible spectroscopy, and Fourier-transform infrared spectroscopy have been used to investigate the optical and structural properties of the films. The results have indicated that a dense, low absorption, and high transparency (in the visible range) waveguide film could be achieved at a low temperature. A strong UV-absorption region at short wavelengths ∼200 nm, accompanied by a shoulder peaked at ∼240 nm, has been noticed due to the neutral oxygen monovacancy defects. The propagation mode and loss properties of the planar waveguide films have also been investigated by using a prism-coupling technique.

Simultaneous measurement of strain and temperature using type I and type IIA fibre Bragg gratings

We present a fibre optic sensing head for making simultaneous measurements of temperature and strain, which operates over a large temperature range. The configuration is based on the different temperature sensitivities of type I and type IIA gratings written in a fibre with high germanium content. Maximal errors of ± 0.7°C Hz−1/2 and ±3.8 µe Hz−1/2 are reported over 500°C and 1200 µe measurement ranges, respectively.

Application of the free volume concept to glasses in the Ge-As-S system

The energy of hole formation Eh, the hole volume Vh, and the fraction fg of fluctuation free volume for glasses in the Ge–As–S system are calculated from the data on the elastic moduli, the microhardness H, and the glass transition temperature Tg. It is shown that the fraction of fluctuation free volume in ternary glasses is considerably larger than that in arsenic chalcogenide glasses. For the glasses studied, the Eh energies fall in the range 13–18 kJ/mol and the hole volumes are equal to (7–84) × 10–6 m3/mol. The concentration dependences of the elastic moduli and the parameters of the free volume theory for glasses in the As2S3–GeS2 system are less pronounced than those for glasses in the As–S system with small coordination numbers of glass-former ions. This is explained by the topological transition observed in glasses of this system at a high germanium content. It is demonstrated that the parameters of the fluctuation free volume theory are related to the characteristics of the boson peak in the Raman spectra.

Synthesis and characterisation of carbon-supported PtGe electrocatalysts for CO oxidation

Platinum–germanium catalysts supported on carbon were synthesised from organometallic precursors using surface organometallic chemistry (SOMC). The catalysts were characterised using chemisorption, X-ray absorption spectroscopy, transmission electron microscopy, energy dispersive X-ray analysis, X-ray diffraction and cyclic voltammetry. Physical characterisation provided evidence for the selective deposition of the germanium with the platinum using SOMC, existing mainly as platinum metal in association with GeO 2. From anodic stripping cyclic voltammetry it can be inferred that the presence of GeO 2 at the platinum sites changes the CO oxidation profile of strongly adsorbed CO, increasing its oxidation at 0.68 V versus RHE relative to the main oxidation at 0.85 V versus RHE. The peak potentials appear to be independent of the germanium loading; however, significant changes were observed in the peak current densities, with an increased promotional effect at higher germanium content. We believe the change in the relative distribution between these sites is mainly due to changes in the surface structure of the platinum on addition of GeO 2. Further promotional activity is seen, with a limited oxidation activity evident at an onset of 0.35 V versus RHE.

Modeling and optimization of thin-film devices with Si/sub 1-x/Ge/sub x/ alloys

Alloys that have a lower bandgap than silicon can extend the infrared response of a silicon cell and hence increase the current generation. One group of materials that are compatible with silicon is Si/sub 1-x/Ge/sub x/ alloys as silicon is completely miscible with germanium. One problem associated with this method is that, because the introduced material has a lower bandgap, it will therefore also cause the device to suffer a loss in voltage. Most research to date has focused on single-junction bulk devices and shows that the loss in voltage overrides the increase in current except for very low germanium content alloys. This work looks at incorporating these Si/sub 1-x/Ge/sub x/ alloys into a thin-film multilayer structure where the flexibility offered through controlling the number and location of junctions facilitates the achievement of high collection probabilities even in thin regions of high germanium concentration where the diffusion lengths are extremely short. PC1D (a one-dimensional circuit simulation package) has been used to simulate the effect of incorporating a layer of Si/sub 1-x/Ge/sub x/ alloy into the multilayer structure. Results show that considerable efficiency enhancement is achieved with this structure, especially for high germanium concentration alloys. The whole range of germanium concentrations is explored.

Composition dependence of surface morphology of ultrathin a-SiGe:H alloys studying by atomic force microscopy

Atomic force microscopy (AFM) has been employed to observe the top surface morphology of hydrogenated amorphous silicon–germanium alloys at the initial stage of deposition. It shows that the different growth mode is dominant for a sample with low and high germanium content, which strongly influences the structures and the photoelectric properties of the corresponding bulk films. The samples with hydrogen plasma annealing are also investigated by AFM in order to understand the effect of hydrogen annealing on the structure of a-SiGe:H films.

SiGe heterojunction bipolar transistors—The noise perspective

Si/SiGe heterojunction transistors with high germanium concentration in the base layer are capable of microwave noise figures below 1 dB at X-band frequencies, because of their low base resistance. We review critical device parameters which influence the minimum noise figure, location of the noise-optimum source reflection coefficient, and associated gain at noise match, including both low-parasitic double-mesa and highly manufacturable box-shaped HBT devices in our considerations. A higher associated gain can be obtained when the base-collector feedback capacitance is reduced. Using larger transistors, both the magnitude of the noise-optimum source reflection coefficient and the equivalent noise resistance will be reduced, making input matching easier and less lossy. Low-frequency noise results show 1/ f corner frequencies below 1 kHz in microwave devices. Two circuit examples, a d.c.-18 GHz amplifier and a low-noise active antenna at 5.8 GHz, demonstrate that device capabilities can be translated into superior circuit performance.

Study of the photosensitivity at 193 nm and comparison with photosensitivity at 240 nm influence of fiber tension: type IIa aging

The photosensitivity in germanosilicate fibers at 193 nm is studied and compared with that at 240 nm. As previously shown, it is found that, without hydrogen sensitization, the initiation of the phenomenon is a one-photon process for a high germanium content fiber and a two-photon process for a low germanium content fiber. However, when the low germanium content fiber is previously loaded with hydrogen, it is observed that the photo-induced index change initial growth rate varies linearly with the fluence, invoking a one-photon process. It is shown that external strains applied to the fiber strongly influence the mechanisms of the photosensitivity at 193 nm. Finally, an aging study is carried out.

Calculation of electron and hole impact ionization coefficients in SiGe alloys

Silicon–germanium alloys offer a system where the ratio of the electron impact ionization coefficient (α) and hole impact ionization coefficient (β) varies from a value larger than unity (in high silicon content alloys), to a value smaller than unity (in high germanium content alloys). We report results for α and β for this alloy system. The electron results are based on a multivalley nonparabolic band structure. The hole results are based on a six-band k⋅p model for low energies coupled to an eight-band model for high energies. We find that for the alloy Si0.4Ge0.6, α∼β. Alloy scattering is found to play an important role in determining the impact ionization coefficient. For compositions around Si0.5Ge0.5, the strong alloy scattering is found to suppress the impact ionization coefficient.