25,062 research outputs found
Fabrication and characterisation of novel Ge MOSFETs
No full textAs high-k dielectrics are introduced into commercial Si CMOS (Complimentary Metal Oxide Semiconductor) microelectronics, the 40 year channel/dielectric partnership of Si/SiO2 is ended and the door opened for silicon to be replaced as the active channel material in MOSFETs (Metal Oxide Semiconductor Field Effect Transistor). Germanium is a good candidate as it has higher bulk carrier mobilities than silicon. In addition, Si and Ge form a thermodynamically stable SiGe alloy of any composition, allowing Ge to be implemented as a thin layer on the surface of a standard Si substrate. This thesis is a practical investigation on several aspects of Ge CMOS technology.
High-k dielectric Ge p-MOSFETs are electrically characterised. A large variation in interface state densities is demonstrated to be responsible for a threshold voltage shift and this is proportional to reciprocal peak mobility due to the Coulomb scattering of carriers by charged states. A theoretical mobility is fitted to that measured at 4.2 K and confirms that interface states are the main source of interface charged impurities.
The model demonstrates a reduction in the interface charged impurity density in p-MOSFETs that underwent a PMA (Post Metallisation Anneal) in hydrogen atmosphere and that the anneal also reduces the RMS (Root Mean Square) dielectric/semiconductor interface roughness, from an average of 0.60 nm to 0.48 nm.
High-k strained Ge p-MOSFETs are electrically characterised and have peak mobilities at 300 K (470 cm2 V-1 s-1) and 4.2 K (1780 cm2 V-1 s-1) far in excess of those measured for the unstrained Ge p-MOSFETs (285 cm2 V-1 s-1,785 cm2 V-1 s-1 respectively). Strained Ge n-MOSFETs perform significantly worse than standard Si P, - MOSFETs primarily due to a high source/drain resistance.
A 10 nm thick SiGe-01 (On Insulator) layer with a Ge composition of 58% is obtained from a 55 nm Si0_88Ge1o2. initial layer on 100 nm Si-Ol substrate via the germanium condensation technique. For the first time, germanium is demonstrated to diffuse through the BOX (Buried OXide) during Ge-condensation and into the underlying Si substrate. An order of magnitude increase in the calculated ITOX (Internal Thermal OXidation) rate of the BOX in the final stages of Ge-condensation is hypothesised to be responsible for stopping this diffusion
Observation of negative differential conductance in a reverse-biased Ni/Ge Schottky diode
No full textWe report the experimental observation of negative differential conductance in a Ni/Ge Schottky diode. With the aid of theoretical models and numerical simulation we show that, at reverse bias, electons tunnel into the high electric field of the depletion region. This scatters the electrons into the upper valley of the Ge conduction band, which has a lower mobility. The observed negative differential conductance is hence attributed to the transferred-electron effect. This shows that Schottky contacts can be used to create hot electrons for transferred-electron devices
Strain-relaxed, high Ge content, SiGe layers grown on Si (100) substrate by reduced pressure - chemical vapour deposition (RP-CVD)
No full textA different approach was taken to relieve strain from a high Germanium (Ge) content,
Silicon-Germanium (SiGe) layers on a Silicon (Si) (100) substrate by growing a thin Ge
under-layer between substrate and layer. The Ge under-layer acts as a strain reliving platform
for further growth of a high Ge content SiGe layer to improve the structural quality of the
sample by reducing the Root Mean Squared Roughness (RRMS) and threading dislocation density
(TDD).
The proposed structure involves the growth of thin Si0.3Ge0.7 and Si0.5Ge0.95 buffer layers of
an average thickness of 350 nm grown on a Si (100) substrate and their structural qualities
assessed. Experimental techniques include High Resolution X-Ray Diffraction, Atomic Force
Microscopy, Transmission Electron Microscopy, and Defect Etching. All samples were
shown to be fully relaxed and have a surface roughness between 1-8 nm. However, a
threading dislocation density of 109 cm-2 was witnessed. Although these results are the first of
their kind, further research into improving structural qualities is to be investigated in the
future
Laser-vibrometric ultrasonic characterization of resonant modes and quality factors of Ge membranes
No full textThe vibrations of a single-crystal germanium (Ge) membrane are studied in air and vacuum using laser vibrometry, in order to determine mechanical properties such as Q-factors, tensile stress, anisotropy, and robustness to shock. Resonance modes up to 3:2 are identified, giving a residual stress measurement of 0.22 GPa, consistent with the value obtained from x-ray relaxation studies. The membrane is found to be isotropic, with Q-factors ranging from around 40 at atmospheric pressure to over 3200 at 5 x 10-4 mbar, significantly lower than those found in polycrystalline Ge micromechanical devices. The robustness to shock is explained through the high resonance mode frequencies and the dissipation mechanism into the substrate, which is a direct consequence of having a high quality film with low residual tensile stress, giving the potential for such films to be used in optoelectronic devices
Ge-on-Si single-photon avalanche diode detectors: design, modeling, fabrication, and characterization at wavelengths 1310 and 1550 nm
No full textThe design, modeling, fabrication, and characterization of single-photon avalanche diode detectors with an epitaxial Ge absorption region grown directly on Si are presented. At 100 K, a single-photon detection efficiency of 4% at 1310 nm wavelength was measured with a dark count rate of ~ 6 megacounts/s, resulting in the lowest reported noise-equivalent power for a Ge-on-Si single-photon avalanche diode detector (1×10-14 WHz-1/2). The first report of 1550 nm wavelength detection efficiency measurements with such a device is presented. A jitter of 300 ps was measured, and preliminary tests on after-pulsing showed only a small increase (a factor of 2) in the normalized dark count rate when the gating frequency was increased from 1 kHz to 1 MHz. These initial results suggest that optimized devices integrated on Si substrates could potentially provide performance comparable to or better than that of many commercially available discrete technologies
Nonconvex multi-period mean-variance portfolio optimization
No full textIn this paper, we address the problem of long-term investment by exploring optimal strategies for allocating wealth among a finite number of assets over multiple periods. Based on the classical Markowitz mean-variance philosophy, we develop a new portfolio optimization framework which can produce sparse portfolios. The sparsity of the portfolio at each and across periods is characterized by the possibly nonconvex penalties. For the constructed nonconvex and nonsmooth constrained model, we propose a generalized alternating direction method of multipliers and its global convergence to a stationary point can be guaranteed theoretically. Moreover, some numerical experiments are conducted on several datasets generated from practical applications to illustrate the effectiveness and advantage of the proposed model and solving method
Atomic structure of the Ge(313) surface
No full textThe atomic structure of the Ge(313)5x1 surface is studied in the present paper. On the basis of high-resolution scanning tunneling microscope images, a detailed structural model of the surface has been proposed. The surface consists of T-4 adatoms, rest atoms, and rebonded atoms, which also exist on other low-index germanium surfaces. The mechanism of the surface reconstruction, however, is different from those of the Ge(111)c(2x8), (001)2x1, (101)c(8x10), and (113)3x1 surfaces.Physics, Condensed MatterSCI(E)8ARTICLE8R4223-R42265
Risk assessment of estuaries under climate change: Lessons from Western Europe
No full textClimate change with rising sea levels and possible changes in surge levels and wave climate will have a large impact on how we protect our coastal areas and cities. Here the focus is on estuarine locations not only affected by tide and surge propagation, but also potentially influenced by freshwater discharge. Mitigation measures might be diverse ranging from pure hard ‘engineering’ solutions all the way to significant realignment. The variation in the type/origin and extent of the flood sources greatly influences subsequent risk management measures. At the same time, society is increasingly demanding that we take a holistic view on risk management, embracing and balancing safety, ecological and socio-economic aspects. This requires that all these diverse factors need to be considered together and integrated. In this context, the Source–Pathway–Receptor (SPR) approach offers a powerful holistic tool to investigate changing risk connected to extreme events.The traditional SPR approach with a consecutive treatment of the flood, pathway and receptor is well understood and is widely used in coastal flood risk analysis. Here an enhanced 2D conceptual version of the SPR method is used to better describe the system and to allow flexibility in considering multiple scales, flood sources and pathways. The new approach is demonstrated by three estuarine case studies in western Europe: the Gironde estuary, France; the Dendermonde region in the Scheldt estuary, Belgium; and HafenCity (Hamburg) in the Elbe estuary, Germany. They differ considerably in the surface area considered, in the type of flood sources, and hence also in the SPR configuration. After a brief introduction of the typical characteristics of the three study sites including some lessons learned from past flood protection measures, the differences in application and results of the SPR approach are discussed. Emphasis is on the specific aspects for each study site, but embedded in a generic SPR framework. The resulting generic lessons learned about the flood sources and how this shapes subsequent analysis are transferable to numerous important estuaries worldwide
Azimuthal correlations in Z +jets events in proton–proton collisions at √s = 13 TeV
No full textAbstract
The production of Z bosons associated with jets is measured in
pp
collisions at
s
=
13
Te
V
with data recorded with the CMS experiment at the LHC corresponding to an integrated luminosity of 36.3
fb
-
1
. The multiplicity of jets with transverse momentum
p
T
>
30
Ge
V
is measured for different regions of the Z boson’s
p
T
(
Z
)
, from lower than 10
Ge
V
to higher than 100
Ge
V
. The azimuthal correlation
Δ
ϕ
between the Z boson and the leading jet, as well as the correlations between the two leading jets are measured in three regions of
p
T
(
Z
)
. The measurements are compared with several predictions at leading and next-to-leading orders, interfaced with parton showers. Predictions based on transverse-momentum dependent parton distributions and corresponding parton showers give a good description of the measurement in the regions where multiple parton interactions and higher jet multiplicities are not important. The effects of multiple parton interactions are shown to be important to correctly describe the measured spectra in the low
p
T
(
Z
)
regions
Atomic structure of the Ge(101) surface
No full textThe atomic structure of the Ge(101)c(8X10) surface is studied in the present paper by means of scanning tunneling microscopy and low-energy electron diffraction. A detailed model of the structure has been proposed for further investigation. The surface consists of zigzag chain atoms, adatoms, dimers, rebonded atoms, and rest atoms arranged at different levels and in different orientations, and thus is even more complicated than the Si(111)7X7 surface.Physics, Condensed MatterSCI(E)31ARTICLE12R6795-R67985
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