A comprehensive two-dimensional VLSI process simulation program, BICEPS

Bell Integrated Circuit Engineering Process Simulator (BICEPS) is a comprehensive VLSI process-simulation program developed at Bell Laboratories. BICEPS incorporates the most up-to-date physical models and efficient numerical algorithms to make it a highly robust and general-purpose program. BICEPS can calculate doping profiles resulting from ion implantation, predeposition, oxidation, and epitaxy in one or two spatial dimensions as well as etching and deposition of oxide, nitride, and photoresist. In this paper, the physics of IC process simulation will be reviewed with an emphasis on the various physical models implemented in BICEPS. Calculation of the impurity profiles in VLSI devices involves the solution of a coupled set of nonlinear time-dependent partial differential equations, with moving boundaries and in more than one spatial dimension. The numerical techniques in obtaining a solution to this problem, namely, spatial discretization, time discretization, and the treatment of moving boundaries are also described in this paper. The capabilities of BICEPS are illustrated by the results of simulation of the fabrication of a typical NMOS transistor.     

Some results on sensitivity and robust stability of input-output systems governed by integral and differential equations

We consider input-output systems (not necessarily of feedback type) on the time domain [0, ) which are governed by nonlinear vector integral or differential equations that relate the input and the output. Assuming that these equations depend on a parameterA, describing perturbations, which is allowed to vary in a vicinity of a nominal valueA ₀ in a linear space, we study how strongly the output is affected by changes of (a)A ₀ when the input is fixed (insensitivity), and (b) the input whenA is fixed withA A ₀ being not too large (robust stability).The results are based on the theory of input-output systems over abstract extended spaces given in Parts I and II of [3]. We discuss nominal systems described by nonlinear Volterra and differential equations, and consider two types of possible perturbations. We also prove a simple result on systems governed by singular differential equations whose perturbations can change the order of the system.     

Spatiotemporal motion boundary detection and motion boundary velocity estimation for tracking moving objects with a moving camera: a level sets PDEs approach with concurrent camera motion compensation

The purpose of this study is to investigate a method of tracking moving objects with a moving camera. This method estimates simultaneously the motion induced by camera movement. The problem is formulated as a Bayesian motion-based partitioning problem in the spatiotemporal domain of the image quence. An energy functional is derived from the Bayesian formulation. The Euler-Lagrange descent equations determine imultaneously an estimate of the image motion field induced by camera motion and an estimate of the spatiotemporal motion undary surface. The Euler-Lagrange equation corresponding to the surface is expressed as a level-set partial differential equation for topology independence and numerically stable implementation. The method can be initialized simply and can track multiple objects with nonsimultaneous motions. Velocities on motion boundaries can be estimated from geometrical properties of the motion boundary. Several examples of experimental verification are given using synthetic and real-image sequences.     

Direct evidence of enhanced chlorine segregation at grain boundaries in polycrystalline CdTe thin films via three‐dimensional TOF‐SIMS imaging

For more than 25 years, the CdTe photovoltaic research and manufacturing communities have been subjecting CdTe materials to a CdCl₂ treatment or activation step to improve performance. However, little work has been carried out using imaging to elucidate the spatial distribution of chlorine in the CdTe devices after this treatment. This work addresses fundamental questions about the spatial distribution of chlorine in the CdTe absorber material after a CdCl₂ treatment comparable to industrial practices. We used a state‐of‐the‐art, time‐of‐flight secondary ion mass spectrometer (ION‐TOF GmbH) (Muenster, Germany) with a lateral resolution of about 80 nm to complete three‐dimensional depth‐profiling and imaging of two CdTe devices. The results clearly demonstrate enhanced chlorine concentration along grain boundaries, supporting the hypothesis that chlorine plays an important role in passivating grain boundaries in CdTe solar cells. The results are discussed in terms of possible passivation mechanisms, and the effect of chlorine on grain interiors and grain boundaries. The data are also used to estimate the free energy of segregation of chlorine to grain boundaries in CdTe. Copyright © 2014 John Wiley & Sons, Ltd.     

Prediction of subharmonic oscillation in switching regulators: from a slope to a ripple standpoint

New exact critical conditions for predicting subharmonic instability in switching regulators are approximated by simple design-oriented expressions valid under practical conditions. These simplified expressions contain the ripple and slope information of the feedback control signal. Depending on the converter topology, the controller used and values of parasitic parameters, either the slope or the ripple can be dominant in predicting instability. A discussion on the validity of this interpretation is illustrated through six different examples of switching regulators using the concept of the spectral radius and the relative degree of the system loop. Using this approach, the boundary between the desired stable region and the subharmonic instability can be easily obtained. The theoretical results are validated by means of numerical simulations.     

Low bias negative differential resistance in C60 dimer modulated by gate voltage

By using a combined method of density functional theory and nonequilibrium Green’s function formalism, we investigate the electronic transport properties of a gated C₆₀ dimer molecule sandwiched between two gold electrodes. The results show that the gate voltage can strongly affect the electronic transport properties of the C₆₀ dimer and change it from semiconducting to metallic. Negative differential resistance behaviors are obtained in such systems and can be modulated to occur at much lower bias by the gate voltage. The low bias negative differential resistance is analyzed from the calculated transmission spectra, projected density of states and the spatial distribution of molecular projected self-consistent Hamiltonian orbitals along with the voltage drop. These results provide a theoretical support to the design of low bias negative differential resistance molecular device by using the modulation of gate voltage.     

Modulation instability of transversely limited electromagnetic waves of terahertz range in strontium titanate paraelectric

Inthe framework of developed approximate phenomenological model we have performed the investigation of modulation instability of transversely limited electromagnetic waves of terahertz range in paraelectric crystals SrTiO₃ at the temperatures of about 77 K. Cubic nonlinearity and frequency dispersion correspond to the existence of modulation instability of long input pulses. The results of numerical investigations of the modulation instability are presented. Transversal boundedness of the input pulses can stabilize the modulation instability. Modulation instability threshold is reduced in the presence of reflections from the crystal boundaries. In the case of development of the modulation instability it is possible to generate a sequence of short terahertz pulses at the output of the crystal. The focusing of input pulses reduces the threshold of modulation instability.     

A hybrid projection method for analysis of waveguide arrays with protruding dielectric elements: A 3D problem

The method proposed for solution of 2D problems is extended to solution of a 3D problem for an array of circular waveguides with protruding cylinder-conical dielectric rods. The transverse electromagnetic field in the domain containing the rods is expanded into a series of vector functions of a Floquet channel. The series contains variable coefficients whose values are determined from solution of a system of ordinary differential equations derived upon projection of the Maxwell equations onto the same vector functions. The differential equations are transformed into algebraic equations via application of a 1D finite-element method and matching of fields with waveguide and spatial harmonics on the boundaries of this domain. The results of numerical solution of the obtained algebraic system are used for calculation of the array characteristics, which are compared to the data published in the literature and the experimental results available for several particular cases.     

Stability of first-order sampling and thyristor systems

Half-order subharmonic instability, which can occur in closed-loop systems containing an ideal sampler feeding a first-order load, does not occur when the sampler is replaced by a thyristor amplifier with the same d.c. gain. Analysis by the subharmonic describing-function method indicates that both systems are stable and so time domain analyses are presented which account for their behaviour.     

A framework for modeling spatial node density in waypoint-based mobility

User mobility is of critical importance when designing mobile networks. In particular, "waypoint" mobility has been widely used as a simple way to describe how humans move. This paper introduces the first modeling framework to model waypoint-based mobility. The proposed framework is simple, yet general enough to model any waypoint-based mobility regimes. It employs first order ordinary differential equations to model the spatial density of participating nodes as a function of (1) the probability of moving between two locations within the geographic region under consideration, and (2) the rate at which nodes leave their current location. We validate our model against real user mobility recorded in GPS traces collected in three different scenarios. Moreover, we show that our modeling framework can be used to analyze the steady-state behavior of spatial node density resulting from a number of synthetic waypoint-based mobility regimes, including the widely used Random Waypoint model. Another contribution of the proposed framework is to show that using the well-known preferential attachment principle to model human mobility exhibits behavior similar to random mobility, where the original spatial node density distribution is not preserved. Finally, as an example application of our framework, we discuss using it to generate steady-state node density distributions to prime mobile network simulations.     

Self-Consistent Solutions for IMPATT Diode Networks

Self-consistent solutions are presented for IMPATT diodes of the Si flat-profile, and GaAs Read types. Particular attention is paid to the onset of subharmonic instability and bias-block oscillations, and the results include numerical verification of a recent analytical theory of subharmonic instability. In addition, a lumped-element realization of the circuit conditions necessary to obtain maximum output power with second-harmonic tuning is described, and its performance is checked by means of self-consistent solution.     

Negative bias temperature instability modeling for high-voltage oxides at different stress temperatures

The temperature bias instability of high-voltage oxides is analyzed. For the investigation of negative bias temperature instability (NBTI) we present an enhanced reaction–diffusion model including trap-controlled transport, the amphoteric nature of the P_b centers at the Si/SiO₂ interface, Fermi-level dependent interface charges, and fully self-consistent coupling to the semiconductor device equations. Comparison to measurement data for a stress/relaxation cycle and a wide range of temperatures shows excellent agreement.     

Blocking of Conducting Channels Widens Window for Ferroelectric Resistive Switching in Interface‐Engineered Hf0.5Zr0.5O2 Tunnel Devices

Films of Hf_0.5Z_0.5O₂ (HZO) contain a network of grain boundaries. In (111) HZO epitaxial films on (001) SrTiO₃, for instance, twinned orthorhombic (o‐HZO) ferroelectric crystallites coexist with grain boundaries between o‐HZO and a residual paraelectric monoclinic (m‐HZO) phase. These grain boundaries contribute to the resistive switching response in addition to the genuine ferroelectric polarization switching and have detrimental effects on device performance. Here, it is shown that, by using suitable nanometric capping layer deposited on HZO film, a radical improvement of the operation window of the tunnel device can be achieved. Crystalline SrTiO₃ and amorphous AlO_x are explored as capping layers. It is observed that these layers conformally coat the HZO surface and allow to increase the yield and homogeneity of ferroelectric junctions while strengthening endurance. Data show that the capping layers block ionic‐like transport channels across grain boundaries. It is suggested that they act as oxygen suppliers to the oxygen‐getters grain boundaries in HZO. In this scenario it could be envisaged that these and other oxides could also be explored and tested for fully compatible CMOS technologies.     

Crystal defects and interdiffusion behavior of Hg1−xCdxTe/(100)CdTe epitaxial layers grown by chemical vapor transport

Crystal defects of chemical vapor transport grown Hg_1−xCd_xTe on (100) CdTe structures have been investigated using chemical etching, wavelength-dispersive spectroscopy, x-ray rocking curve, and scanning electron microscopy methods The results indicate that the origin and spatial distribution of the misfit dislocations can be attributed to both the lattice parameter misfit and the inevitable interdiffusion occurring between the substrate and the epitaxial layer. It is proposed that the interdiffusion of Hg along the [100] direction is enhanced by dislocation channels and other defect cores along or near this direction owing to defects on the initial surface of the CdTe substrate. The results indicate that the subgrain boundaries in Hg_1−xCd_xTe are caused by slight misorientation of the lattices and polygonization of the defects during epitaxial layer growth, and by the propagation of the subgrain boundaries existing in the CdTe substrate.     

Negative-mass instability at nonsymmetrical perturbations

The negative-mass (space-charge) instability is studied within the model of a flat thin electron beam moving along the stationary uniform magnetic field in the case of nonsymmetrical perturbations. It is shown that due to the phenomenon of "phase mixing" of electron bunches the instability increments are small for perturbations with spatial scales smaller than the Larmor diameter.     

Single-mode fast-tunable lasers for laser-diode spectroscopy

Physical mechanisms leading to the suppression of instability in the electron-hole plasma under conditions of population inversion and, thus, promoting single-mode lasing and drive-current-controlled tuning were studied; in particular, nonuniform injection and spatial oscillations of laser flux were considered. Transient times typical of current-and heat-aided tuning are measured. The effective time constant is estimated as ∼1 μs for heat-controlled tuning; by contrast, it is at least one order of magnitude shorter for current tuning. The tuning range does not exceed several angstroms and is as wide as 100 ? in the case of heat-and current-aided tuning, respectively. A single-mode fast-tunable heterolaser which is capable of operating within the 2.8–3.6 μm wavelength range at 12–120 K and is designed for laser-diode spectroscopy is developed. The results of using the laser for the detection of absorption spectra in OCS, NH₃, CH₃Cl, CH₄, N₂O, and H₂O vapors are reported. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 8, 2003, pp. 985–995. Original Russian Text Copyright ? 2003 by Astakhova, Danilova, Imenkov, Kolchanova, Yakovlev.     

Segregation of phosphorus and germanium to grain boundaries in chemical vapor deposited silicon-germanium films determined by scanning transmission electron microscopy

The segregation of phosphorus and germanium to grain boundaries in P implanted, Si_0.87Ge_0.13 films deposited by chemical vapor deposition (CVD), was investigated using energy dispersive x-ray (EDX) micro-analysis. A quantitative analysis of the x-ray spectra obtained at grain boundaries showed that the excess amount of P varied with the crystallography of the boundary but that the segregation always followed an equilibrium process with an activation energy of 0.28 eV. On the other hand, Ge did not segregate to grain boundaries in either P implanted Si_0.87Ge_0.13 films nor in intrinsic Si_1−xGe_x films, containing 2, 13 and 31 at.% Ge. Possible reasons for the absence of Ge segregation are discussed.     

Enhancing Resistive Switching Performance and Ambient Stability of Hybrid Perovskite Single Crystals via Embedding Colloidal Quantum Dots

Hybrid organic‐inorganic halide perovskites are actively pursued for optoelectronic technologies, but the poor stability is the Achilles’ heel of these materials that hinders their applications. Very recently, it has been shown that lead sulfide (PbS) quantum dots (QDs) can form epitaxial interfaces with the perovskite matrix and enhance the overall stability. In this work, it is demonstrated that embedding QDs can significantly modify the transport property of pristine perovskite single crystals, endowing them with new functionalities besides being structurally robust and free from grain boundaries. Resistive switching memory devices are constructed using solution‐processed CH₃NH₃PbBr₃ (MAPbBr₃) perovskite single crystals and the QD‐embedded counterparts. It is found that QDs could significantly enhance the charge transport and reduce the current–voltage hysteresis. The pristine singe crystal device exhibits negative differential resistance, while the QD‐embedded crystals are featured with filament‐type switching behavior and much improved device stability. This study underscores the potential of QD‐embedded hybrid perovskites as a new media for advanced electronic devices.     

Heteroatom‐Modulated Switching of Photocatalytic Hydrogen and Oxygen Evolution Preferences of Anatase TiO2 Microspheres

Understanding and manipulating the two half‐reactions of photoinduced electron reduction and hole oxidation are key to designing and constructing efficient photocatalysts. Here, how the spatial distribution of the heteroatom modulates photocatalytic reduction (hydrogen evolution) and oxidation (oxygen evolution) reaction preferences is investigated by moving boron from the core to the shell of an anatase TiO₂ microsphere along [001] via thermal diffusion control. The preference towards photocatalytic hydrogen and oxygen producing reactions from splitting water can be switched by creating a shell with an interstitial B^σ+ (σ ≤ 3) gradient in the TiO₂ microsphere. This switching stems from the downward shift of electronic band edges of the shell by a band bending effect that originates from the extra electrons coming from the interstitial B^σ+. These results create new opportunities for designing and constructing efficient photocatalysts by spatial heteroatom engineering.     

Ein modell des korngrenzenwiderstandes in dotierter BaTiO3-keramik

Current-voltage characteristics and the dependence of the differential capacitance on the bias voltage are calculated for the high-resistance grain boundaries in doped BaTiO₃ ceramics. A good adaption to the experimental results can be achieved. The influence and the values of the parameters used for the calculation are discussed.