Low Al-content n-type AlxGa1-xN layers with a high-electron-mobility grown by hot-wall metalorganic chemical vapor deposition

In this work, we demonstrate the capability of the hot-wall metalorganic\ua0chemical vapor deposition\ua0to deliver high-quality\ua0n-AlxGa1−xN (x\ua0= 0\ua0–\ua00.12, [Si] = 1 71017\ua0cm−3)\ua0epitaxial layers\ua0on 4H-SiC(0001). All layers are crack-free, with a very small root mean square roughness (0.13\ua0–\ua00.25 nm), homogeneous distribution of Al over film thickness and a very low unintentional incorporation of oxygen at the detection limit of 5 71015\ua0cm−3\ua0and carbon of 2 71016\ua0cm−3. Edge type dislocations in the layers gradually increase with increasing Al content while\ua0screw dislocations\ua0only raise for\ua0x\ua0above 0.077. The room temperature\ua0electron mobility\ua0of the\ua0n-AlxGa1−xN remain in the range of 400\ua0–\ua0470 cm2/(V.s) for Al contents between 0.05 and 0.077 resulting in comparable or higher Baliga figure of merit with respect to GaN, and hence demonstrating their suitability for implementation as drift layers in power device applications. Further increase in Al content is found to result in significant deterioration of the electrical properties

Pricing and Hedging Asian Basket Options with Quasi-Monte Carlo Simulations

In this article we consider the problem of pricing and hedging high-dimensional Asian basket options by Quasi-Monte Carlo simulation. We assume a Black-Scholes market with time-dependent volatilities and show how to compute the deltas by the aid of the Malliavin Calculus, extending the procedure employed by Montero and Kohatsu-Higa (2003). Efficient path-generation algorithms, such as Linear Transformation and Principal Component Analysis, exhibit a high computational cost in a market with time-dependent volatilities. We present a new and fast Cholesky algorithm for block matrices that makes the Linear Transformation even more convenient. Moreover, we propose a new-path generation technique based on a Kronecker Product Approximation. This construction returns the same accuracy of the Linear Transformation used for the computation of the deltas and the prices in the case of correlated asset returns while requiring a lower computational time. All these techniques can be easily employed for stochastic volatility models based on the mixture of multi-dimensional dynamics introduced by Brigo et al. (2004).Comment: 16 page

Quasi-Monte Carlo, Discrepancies and Error Estimates

We discuss the problem of defining an estimate for the error in quasi-Monte Carlo integration. The key issue is the definition of an ensemble of quasi-random point sets that, on the one hand, includes a sufficiency of equivalent point sets, and on the other hand uses information on the degree of uniformity of the point set actually used, in the form of a discrepancy or diaphony. A few examples of such discrepancies are given. We derive the distribution of our error estimate in the limit of large number of points. In many cases, Gaussian central limits are obtained. We also present numerical results for the quadratic star-discrepancy for a number of quasi-random sequences

Achievable information rates estimation for 100-nm ramana amplified optical transmission system

The achievable information rates of optical communication systems using ultra-wide bandwidth 100-nm distributed Raman amplification have been investigated for each individual subchannels, based on the first-order perturbative analysis of nonlinear distortions

Effect of high-temperature annealing on the residual strain and bending of freestanding GaN films grown by hydride vapor phase epitaxy

The effect of high-temperature high-pressure annealing on the residual strain, bending, and point defect redistribution of freestanding hydride vapor phase epitaxial GaN films was studied. The bending was found to be determined by the difference in the in-plane lattice parameters in the two faces of the films. The results showed a tendency of equalizing the lattice parameters in the two faces with increasing annealing temperature, leading to uniform strain distribution across the film thickness. A nonmonotonic behavior of structural parameters with increasing annealing temperature was revealed and related to the change in the point defect content under the high-temperature treatment.Peer reviewe

Responding to COVID‐19 through Surveys of Public Servants

Responding to COVID‐19 presents unprecedented challenges for public sector practitioners. Addressing those challenges requires knowledge about the problems that public sector workers face. This Viewpoint essay argues that timely, up‐to‐date surveys of public sector workers are essential tools for identifying problems, resolving bottlenecks, and enabling public sector workers to operate effectively during and in response to the challenges posed by the pandemic. This essay presents the COVID‐19 Survey of Public Servants, which is currently being rolled out in several countries by the Global Survey of Public Servants Consortium to assist governments in strategically compiling evidence to operate effectively during the COVID‐19 pandemic

Tuning composition in graded AlGaN channel HEMTs toward improved linearity for low-noise radio-frequency amplifiers

Compositionally graded channel AlGaN/GaN high electron mobility transistors (HEMTs) offer a promising route to improve device linearity, which is necessary for low-noise radio-frequency amplifiers. In this work, we demonstrate different grading profiles of a 10-nm-thick AlxGa1-xN channel from x = 0 to x = 0.1 using hot-wall metal-organic chemical vapor deposition (MOCVD). The growth process is developed by optimizing the channel grading and the channel-to-barrier transition. For this purpose, the Al-profiles and the interface sharpness, as determined from scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, are correlated with specific MOCVD process parameters. The results are linked to the channel properties (electron density, electron mobility, and sheet resistance) obtained by contactless Hall and terahertz optical Hall effect measurements coupled with simulations from solving self-consistently Poisson and Schr\uf6dinger equations. The impact of incorporating a thin AlN interlayer between the graded channel and the barrier layer on the HEMT properties is investigated and discussed. The optimized graded channel HEMT structure is found to have similarly high electron density (∼9 7 10 12 cm-2) as the non-graded conventional structure, though the mobility drops from ∼ 2360 cm2/V s in the conventional to ∼ 960 cm2/V s in the graded structure. The transconductance gm of the linearly graded channel HEMTs is shown to be flatter with smaller g m ′ and g m ″ as compared to the conventional non-graded channel HEMT implying improved device linearity