Oblique stacking of three-dimensional dome islands in Ge/Si multilayers

The organization of Ge "dome" islands in Ge/Si multilayers has been investigated by cross-sectional transmission electron microscopy. Ge domes are found to spontaneously arrange in oblique stacks, replicating at a well-defined angle from one bilayer to the next. The formation of oblique island stacks is governed by a complex interplay of surface strain, generated by the already buried islands, and surface curvature, caused by the inherent tendency of large domes to carve out material from the surrounding planar substrate. (C) 2001 American Institute of Physics

Replicating Nanostructures on Silicon by Low Energy Ion Beams

We report on a nanoscale patterning method on Si substrates using self-assembled metal islands and low-energy ion-beam irradiation. The Si nanostructures produced on the Si substrate have a one-to-one correspondence with the self-assembled metal (Ag, Au, Pt) nanoislands initially grown on the substrate. The surface morphology and the structure of the irradiated surface were studied by high-resolution transmission electron microscopy (HRTEM). TEM images of ion-beam irradiated samples show the formation of sawtooth-like structures on Si. Removing metal islands and the ion-beam induced amorphous Si by etching, we obtain a crystalline nanostructure of Si. The smallest structures emit red light when exposed to a UV light. The size of the nanostructures on Si is governed by the size of the self-assembled metal nanoparticles grown on the substrate for this replica nanopatterning. The method can easily be extended for tuning the size of the Si nanostructures by the proper choice of the metal nanoparticles and the ion energy in ion-irradiation. It is suggested that off-normal irradiation can also be used for tuning the size of the nanostructures.Comment: 12 pages, 7 figures, regular paper submitted to Nanotechnolog

Extraction of the active acceptor concentration in (pseudo-) vertical GaN MOSFETs using the body-bias effect

We report and discuss the performance of an enhancement mode n-channel pseudo-vertical GaN metal oxide semiconductor field effect transistor (MOSFET). The trench gate structure of the MOSFET is uniformly covered with an Al₂O₃ dielectric and TiN electrode material, both deposited by atomic layer deposition (ALD). Normally-off device operation is demonstrated in the transfer characteristics. Special attention is given to the estimation of the active acceptor concentration in the Mg doped body layer of the device, which is crucial for the prediction of the threshold voltage in terms of device design. A method to estimate the electrically active dopant concentration by applying a body bias is presented. The method can be used for both pseudo-vertical and truly vertical devices. Since it does not depend on fixed charges near the channel region, this method is advantageous compared to the estimation of the active doping concentration from the absolute value of the threshold voltage

Local scale invariance and strongly anisotropic equilibrium critical systems

A new set of infinitesimal transformations generalizing scale invariance for strongly anisotropic critical systems is considered. It is shown that such a generalization is possible if the anisotropy exponent \theta =2/N, with N=1,2,3 ... Differential equations for the two-point function are derived and explicitly solved for all values of N. Known special cases are conformal invariance (N=2) and Schr\"odinger invariance (N=1). For N=4 and N=6, the results contain as special cases the exactly known scaling forms obtained for the spin-spin correlation function in the axial next nearest neighbor spherical (ANNNS) model at its Lifshitz points of first and second order.Comment: 4 pages Revtex, no figures, with file multicol.sty, to appear in PR

Multi-particle structure in the Z_n-chiral Potts models

We calculate the lowest translationally invariant levels of the Z_3- and Z_4-symmetrical chiral Potts quantum chains, using numerical diagonalization of the hamiltonian for N <= 12 and N <= 10 sites, respectively, and extrapolating N to infinity. In the high-temperature massive phase we find that the pattern of the low-lying zero momentum levels can be explained assuming the existence of n-1 particles carrying Z_n-charges Q = 1, ... , n-1 (mass m_Q), and their scattering states. In the superintegrable case the masses of the n-1 particles become proportional to their respective charges: m_Q = Q m_1. Exponential convergence in N is observed for the single particle gaps, while power convergence is seen for the scattering levels. We also verify that qualitatively the same pattern appears for the self-dual and integrable cases. For general Z_n we show that the energy-momentum relations of the particles show a parity non-conservation asymmetry which for very high temperatures is exclusive due to the presence of a macroscopic momentum P_m=(1-2Q/n)/\phi, where \phi is the chiral angle and Q is the Z_n-charge of the respective particle.Comment: 22 pages (LaTeX) plus 5 figures (included as PostScript), BONN-HE-92-3

Miniband-related 1.4–1.8 μm luminescence of Ge/Si quantum dot superlattices

The luminescence properties of highly strained, Sb-doped Ge/Si multi-layer heterostructures with incorporated Ge quantum dots (QDs) are studied. Calculations of the electronic band structure and luminescence measurements prove the existence of an electron miniband within the columns of the QDs. Miniband formation results in a conversion of the indirect to a quasi-direct excitons takes place. The optical transitions between electron states within the miniband and hole states within QDs are responsible for an intense luminescence in the 1.4–1.8 µm range, which is maintained up to room temperature. At 300 K, a light emitting diode based on such Ge/Si QD superlattices demonstrates an external quantum efficiency of 0.04% at a wavelength of 1.55 µm

An Algorithm Informed by the Parathyroid Hormone Level Reduces Hypocalcemic Complications of Thyroidectomy

Ó The Author(s) 2010. This article is published with open access at Springerlink.com Background Measurement of the parathyroid hormone (PTH) level following total thyroidectomy (TTx) may allow prediction of postoperative hypocalcemia. We present an algorithmic method of managing hypocalcemia preemptively, based on the PTH level 1 h after operation. Materials and methods We examined 423 consecutive patients undergoing TTx at a single institution. A subset of patients were managed using an algorithm involving routine postoperative oral calcium administration and the early addition of oral calcitriol in patients with a low 1-h postoperative PTH level. Algorithm patients were compared to a concurrent, conventionally managed group. Outcomes measured included serum calcium levels, symptoms of hypocalcemia, postoperative complications, and receipt of intravenous (IV) calcium. Results The algorithm was applied in 135 patients, and 288 patients were managed conventionally. Critically low calcium levels (total calcium \7.5 mg/dl [1.88 mmol/l] or ionized calcium \0.94 mmol/l) were less common in algorithm patients (10.6 % vs. 25.3%; p \ 0.005). Much of this difference was attributable to the protective impact of the algorithm on patients undergoing TTx for cancer, 30% of whom developed critically low calcium levels when managed conventionally. Among patients requiring IV calcium, algorithm patients received fewer doses (1.29 vs

ICAR: endoscopic skull‐base surgery

n/

Line shape analysis of electron hole plasma electroluminescence in fully strained SiGe epitaxial layers

The electroluminescence of p-i-n diodes with fully strained Si0.80Ge0.20/Si(001) is dominated by radiative recombination in an electron-hole plasma. The recombination mechanisms and the band gap renormalization have been studied experimentally and by modeling. In order to minimize the influence of the SiGe/Si interface regions and thus to study the intrinsic behavior of strained SiGe, electroluminescence diodes with thick layers of SiGe in a metastable strain state have been investigated. To explain the electroluminescence spectra, the band filling model for an electron-hole plasma system and different broadening procedures have been investigated. This line shape analysis allowed the determination of the dependence of the renormalized band gap on carrier density and comparison with theoretical predictions was done. The low-energy tails of the electroluminescence spectra correspond to a broadening of the initial electronic states of the recombination process in electron-hole plasma system. The experimental data obtained from the line shape analysis of the electroluminescence spectra are in good agreement with previous results on spectral photocurrent and quantum efficiency measurements. (C) 2003 American Institute of Physics

Optoelectronic properties of thick SiGe layers grown as small mesas by low pressure chemical vapor deposition

Arrays of Si0.80Ge0.20/Si(001) square mesas were epitaxially grown by low pressure chemical vapor deposition to optimize the light emission in the near infrared range. To study the influence of mesa size on light emission the current-voltage characteristics, the spectral photocurrent, and the electroluminescence of p-i-n structures were measured. While the plastic relaxation has a strong influence on the electroluminescence spectra, the current-voltage characteristics are only slightly changed. At low temperatures, a tunneling current was observed and its possible location is discussed. Due to the high SiGe thickness, both the contributions of the no-phonon and transversal optical phonon-assisted transitions to the photocurrent spectra could be observed. Direct evidence of the higher band gap of relaxed SiGe was obtained from electroluminescence studies. (C) 2003 American Institute of Physics