Simulation study of circuit performances of independent double-gate (IDG) MOSFETs with high-permittivity gate dielectrics

This study analyzes by 2-D numerical simulation the electrical performances of independent double-gate (IDG) MOSFET with high-permittivity (high-κ) dielectric gate stacks. We particularly address the operation of elementary devices as well as of the associated CMOS inverters, which are investigated in terms of static power dissipation and delay. We show that the introduction of a pure high-κ gate dielectric layer degrades the device immunity to short-channel effects and the power consumption of the CMOS inverter, but simultaneously improves the inverter speed, with respect to performances of a reference transistor with a SiO₂ gate oxide with the same equivalent oxide thickness. However, in real devices, the existence of a native oxide between the high-κ gate dielectric and the underlying semiconductor reduces parasitic electrostatic effects and sensibly improves inverter speed and power consumption. A detailed comparison between the operation of high-κ gate stack-based double-gate devices with independent and connected gates is also presented.     

Determination of the contribution of defect creation and charge trapping to the degradation of a-Si:H/SiN TFTs at room temperature and low voltages

In this paper is presented a qualitative investigation upon the mechanisms that cause the shift of the electrical parameters in thin film transistors (TFT). The transistors are made of amorphous hydrogenated silicon (a-Si:H) as active semiconductor layer and substoichiometric silicon nitride (SiN) as gate insulator. The work refers to the degradation at room temperature and low positive and negative gate voltage stresses. The electrical parameters: threshold voltage (V_th), subthreshold swing (S) and flat-band voltage (V_fb) have been determined from the dependence of the drain current on gate voltage (I_d–V_g) and the dependence of the capacitance on the gate voltage (C–V_g) measurements as function of stress time and bias.     

Investigation of capacitance–voltage characteristics in Ge /high-κ MOS devices

We developed a one-dimensional numerical simulation code for the calculation of the gate voltage–capacitance characteristic of MOS structures including the self-consistently solving of the Schrödinger and Poisson equations for different alternative channel materials with high mobility such as Ge, and non-conventional gate dielectrics such as HfO₂ and Al₂O₃. Our simulation results are confronted to experimental data for various MOS structures with different semiconductors and dielectric stacks.     

Self-formation of dielectric layer containing CoSi2 nanocrystals by plasma-enhanced atomic layer deposition

Dielectric layer containing CoSi₂ nanocrystals was directly fabricated by plasma-enhanced atomic layer deposition using CoCp₂ and NH₃ plasma mixed with SiH₄ without annealing process. Synchrotron radiation X-ray diffraction and X-ray photoelectron spectroscopy results confirmed the formation of CoSi₂ nanocrystal. The gate stack composed of dielectric layer containing CoSi₂ nanocrystals with ALD HfO₂ capping layer together with Ru metal gate was analyzed by capacitance–voltage (C–V) measurement. Large hysteresis of C–V curves indicated charge trap effects of CoSi₂ nanocrystals. The current process provides simple route for the fabrication of nanocrystal memory compatible with the current Si device unit processes.     

Characterization of silicon layers epitaxially grown on metallurgical-grade polycrystalline substrates

The structural and electrical properties of silicon layers epitaxially grown on metallurgical-grade polycrystalline silicon substrates are examined to clarify the effect of grain boundaries, crystal defects and impurities in the substrates. Chemical etching of the epitaxial layer reveals that all the grain boundaries continue from the substrate into the epitaxial layer, whereas lines of high density etch pits do not always continue. The polycrystalline thin film solar cells are fabricated on the metallurgical-grade silicon substrates by successive deposition of p and n⁺ layers. These cells show short circuit current densities around 70% of that of the conventional single crystal cell. This reduction of the short circuit current is caused mainly by the short minority carrier diffusion length in the grains probably due to impurities involved in the epitaxial layers. The origins of such impurities are discussed by considering autodoping and solid-state diffusion from the substrate during growth of epitaxial layers.     

A new physical-based compact model of floating-gate EEPROM cells

A model for static and transient simulations of an electrically erasable programmable read-only memory (EEPROM) cell has been developed. This physical compact model is based on charge sheet approach which is able to describe the complete electrical properties of the cell. In this model the charge neutrality, including the charge trapped on the floating gate, is applied to determine the surface potential. From the surface potential, related to the terminal voltages, the drain current and the different charges present in the cell structure can be calculated. This model has been successfully implemented in common circuit simulators (Eldo and Saber) and used for the study of the write/erase operations in an EEPROM cell.     

Microscopic study of the H2O vapor treatment of the silicon grain boundaries

We have proposed annealing in H₂O vapor as a new effective and low-cost method for passivating polycrystalline silicon grain boundaries and for improving the performance of poly-Si based devices. The effect of H₂O vapor treatment was experimentally found to differ from analogous anneal in nitrogen and hydrogen. Mechanism of the H₂O vapor treatment was studied by Kelvin force microscopy, used to measure the potential change at individual grain boundaries and point defects. The potential change was dependent on the grain boundary character and it correlated with the crystalline disorder and internal stress observed by microscopic Raman spectroscopy. Effect of H₂O vapor passivation was experimentally connected with the potential change at the grain boundaries before and after the treatment.     

A new approach to thin film crystal silicon on glass: Biaxially-textured silicon on foreign template layers

We propose a new approach to growing photovoltaic-quality crystal silicon films on glass. Other approaches to film Si focus on increasing grain size in order to reduce the deleterious effects of grain boundaries. Instead, we propose aligning the silicon grains biaxially (both in and out of plane) so that (1) grain boundaries are ‘low-angle’ and have less effect on the electronic properties of the material and (2) subsequent epitaxial thickening is simplified. They key to our approach is the use of a foreign template layer that can be grown with biaxial texture directly on glass by a technique such as ion-beam-assisted deposition or inclined substrate deposition. After deposition of the template layer, silicon is then grown aligned to the template and subsequently thickened. Here, we outline this new approach to silicon on glass, describe initial experimental results and discuss challenges that must be overcome.     

Structural and electrical characterization of ALCVD ZrO2 thin films on silicon

We report on the structural and electrical properties of ZrO₂ thin layers grown on Si by atomic layer chemical vapour deposition. Atomic force microscopy, X-ray diffraction, X-ray reflectivity and time-of-flight secondary ion mass spectrometry have been used to characterize as-grown and annealed samples. High frequency capacitance-voltage measurements have been performed to determine the capacitance of the gate dielectric stack. The ZrO₂ film is found to be polycrystalline. Electrical and structural data suggest a coherent picture of film modification upon annealing.     

Design of exposed-core fiber for methadone monitoring in biological fluids

In this paper, the design of a silica exposed-core fiber sensor for methadone detection in water is reported. The sensor feasibility is numerically investigated using a Finite Element Method (FEM) numerical code. The simulation results have evaluated the fiber cross sections that incorporate a polymeric cladding as a sensitive layer as a means of enhancing the evanescent field interaction with the polymeric layer permeated by the analyte. In addition to the proposed biomedical sensor, a similar sensing configuration is proposed and modeled for environment pollution. Lead silicate glass (F2) is considered in the simulation as an alternative to silica glass and different contaminants (benzene, toluene etc.) are taken into account to evaluate the sensor suitability for use in environment monitoring.     

Effect of growth conditions on formation of grain boundaries in epitaxial silicon layers

The formation of grain boundaries of the general type, along with small-and large-angle symmetric grain boundaries with the 〈 〉 axis in the epitaxial layers grown onto bicrystal substrates by the method of thermal migration has been studied. The solvent was aluminum. It is shown that if the grain boundaries in the epitaxial layer are tilted to the crystallization front or if there is a temperature gradient tangential to this front, their orientation differs from their orientation in the substrate. The large-angle symmetric boundaries are more stable than the boundaries of the general type. The grain-boundary energy and rotation moment of large-angle symmetric boundaries are evaluated.     

Three dimensional simulation of melt flow in Czochralski crystal growth with steady magnetic fields

Three-dimensional transient numerical simulations were carried out to investigate the melt convection and temperature fluctuations within an industrial Czochralski crucible. To study the magnetic damping effects on the growth process, a vertical magnetic field and a cusp magnetic field were considered. Due to our special interest in the melt convection, only local simulation was conducted. The melt flow was calculated by large-eddy simulation (LES) and the magnetic forces were implemented in the CFD code by solving a set of user-defined scalar (UDS) functions. In the absence of magnetic fields, the numerical results show that the buoyant plumes rise from the crucible to the free surface and the crystal–melt interface, which indicates that the heat and mass transfer phenomena in Si melt can be characterized by the turbulent flow patterns. In the presence of a vertical magnetic field, the temperature fluctuations in the melt are significantly damped, with the buoyant plumes forming regular cylindrical geometries. The cusp magnetic field could also markedly reduce the temperature fluctuations, but the buoyant plumes would break into smaller vortical structures, which gather around the crystal as well as in the center of the crucible bottom. With the present crucible configurations, it is found that the vertical magnetic field with an intensity of 128 mT can damp the temperature fluctuations more effectively than the 40 mT cusp magnetic field, especially in the region near the growing crystal.     

Effect of some technological parameters on Fowler–Nordheim injection through tunnel oxides for non-volatile memories

In this work the effects of various technological parameters on Fowler–Nordheim injection through thin (around 7.2 nm) silicon dioxide films in metal-oxide-semiconductor capacitors have been studied. Attention has been paid to the effect of gate geometry (round or strip gate) and area, substrate doping type (boron or phosphorus one), polycrystalline silicon gate structure (simple polysilicon or polysilicon-oxide-nitride-oxide-polysilicon structure) and tunnel oxide type (standard or nitrided silicon dioxide). The effect of all these parameters on Fowler–Nordheim tunneling injection and on the potential barrier height at both oxide injecting interfaces are usually neglected in literature and moreover the tunnel coefficients obtained from a simple capacitor are used in the simulation of programmable operations of electrically erasable programmable read only memories. Quasi-static capacitance (voltage) and current (voltage) measurements have been performed and the latter have been simulated by using a constant effective barrier height at the injecting interface. We have found that Fowler–Nordheim tunneling parameters and potential barrier height at both oxide injecting interfaces are affected by the substrate doping type, oxide type, gate geometry and gate structure but they are not affected by the gate area. Moreover in all structures, a difference between the barrier heights at the two injecting interfaces has been observed. The variation induced by the studied technological parameters on the potential barrier height are comparable to the variation induced by considering a constant (classical theory) or electrical field dependent (quantum theory) barrier height as reported in literature.     

Evaluation of charge trapping properties of microcrystalline germanium thin films by Kelvin force microscopy

We have prepared highly-crystallized germanium (Ge) films on quartz and evaluated their local charge trapping and electrical conduction properties from topographic and surface potential images simultaneously taken by a conductive atomic force microscopy (AFM) during and after current application to Ge films. By applying a bias of 10 V at which the current of ~ 8 mA flows between the co-planer electrodes on Ge films, the surface potential image which was uniform before bias application shows in-plane inhomogeneity within ~ 1.0 mV commensurate with the surface morphology. Such potential images remained inhomogeneous at zero bias for more than two hours after bias application. The inhomogeneous potential images can be interpreted in terms of the difference in electron concentration in highly-crystallized Ge films presumably caused by electron charging in the grain boundaries, indicating direct detection of electrically separated grain structures and resultant percolation current pass.     

Design of silica-based photonic crystal fiber for biosensing applications

In this paper, the design of a new photonic crystal fiber sensor for methadone detection in water is reported. The sensor feasibility is numerically demonstrated via accurate simulations performed by employing a Finite Element Method (FEM) numerical code. The fiber cross section, including a polymeric cladding as sensitive layer, is optimized. In particular, the sensor refinement is obtained via a large number of simulations performed by varying the thickness of the polymeric layer, the hole diameter, the hole-to-hole spacing and the transversal distribution of the holes. The simulation results suggest that a suitable fiber cross section design can enhance the evanescent field interaction with the polymeric layer permeated by the analyte.     

Characterisation of ALCVD Al2O3-ZrO2 nanolaminates, link between electrical and structural properties

Al₂O₃ and ZrO₂ mixtures for gate dielectrics have been investigated as replacements for silicon dioxide aiming to reduce the gate leakage current and reliability in future CMOS devices. Al₂O₃ and ZrO₂ films were deposited by atomic layer chemical vapor deposition (ALCVD) on HF dipped silicon wafers. The growth behavior has been characterized structurally and electrically. ALCVD growth of ZrO₂ on a hydrogen terminated silicon surface yields films with deteriorated electrical properties due to the uncontrolled formation of interfacial oxide while decent interfaces are obtained in the case of Al₂O₃. Another concern with respect to reliability aspects is the relatively low crystallization temperature of amorphous high-k materials deposited by ALCVD. In order to maintain the amorphous structure at high temperatures needed for dopant activation in the source drain regions of CMOS devices, binary Al/Zr compounds and laminated stacks of thin Al₂O₃ and ZrO₂ films were deposited. X-ray diffraction and transmission electron microscope analysis show that the crystallization temperature can be increased dramatically by using a mixed oxide approach. Electrical characterization shows orders of leakage current reduction at 1.1-1.7 nm of equivalent oxide thickness. The permittivity of the deposited films is determined by combining quantum mechanically corrected capacitance voltage measurements with structural analysis by transmission electron microscope, X-ray reflectivity, Rutherford backscattering, X-ray photoelectron spectroscopy, and inductively coupled plasma optical emission spectroscopy. The k-values are discussed with respect to formation of interfacial oxide and possible silicate formation.     

Wave-mechanical study of gate tunneling leakage reduction in ultra-thin (<2 nm) dielectric MOS and H-MOS devices

The downsizing of the metal oxide semiconductor field effect transistor (MOSFET) to dimensions <0.1 μm in the next years requires the adoption of <2 nm thick gate oxides, which unfortunately give rise to a significant direct tunneling current. We apply in this paper a self-consistent one-dimensional Schrödinger–Poisson model to study MOSFET channel architectures likely to reduce this leakage. They consist in either increasing the insulator thickness using a larger permittivity nitride/oxide stack or burying the channel thanks to a tensile strained IV–IV heterostructure. It is shown that both solutions yield separately a reduction, and that the combination of these two strategies offers promising opportunities to fulfill ultimate complementary metal oxide semiconductor technology (0.1 μm) requirements regarding gate oxide downsizing.     

On the recrystallization of metals in the emission electron microscope

By the emission electron microscope a high-contrast picture of grain structure is obtained with thermal or photoelectric release of electrons. The „memory effect”︁ (MORLIN , TREMMEL ) marks the individual steps of the grain boundary movement through volume expansion or thermal etching when releasing electrons by electrons or ions. The existence of an oxide layer on the surface of e.g. molybdenum and tungsten is starting the recrystallization and influencing the recrystallization temperature. Oxide layers are formed in selected areas corresponding to the distribution of surface seeds and the oxygen bombardment. Carbon atoms accumulate preferably at the grain boundaries as carbides, so decorating the crystallites in forming mixed crystals. – It may be assumed that the carbon solved in tungsten and molybdenum fixes the dislocations at the grain boundaries. In contradiction to all theories of secondary recrystallization „ring crystals”︁ are formed.     

金刚石衬底上GaN HEMT沟道温度建模:各向异性且非均匀热导率的影响

为了改善GaN HEMT的自热效应,集成高热导率的金刚石衬底有助于增强器件有源区的热量耗散。然而,化学气相淀积(CVD)生长的多晶金刚石(PCD)具有柱状晶粒结构,导致了各向异性的材料热导率,且其热导率值与生长厚度有关。为此,通过建模金刚石生长过程中晶粒尺寸的演变过程,计算了金刚石沿面内和截面方向的热导率。基于该PCD热导率模型,利用计入材料非线性热导率的GaN器件热阻解析模型,计算得到了GaN HEMT沟道温度的波动范围,并分析了其与器件结构(栅长、栅宽、栅间距、衬底厚度)和功耗的依赖关系。最后,通过与有限元(FEM)仿真结果对比,分区域提取了GaN HEMT器件中PCD衬底的有效热导率,分别为260~310 W/(m·K)和1 250~1 450 W/(m·K)。本文的计算为预测金刚石衬底上GaN HEMT器件的沟道温度提供了快速、有效的方法。     

Recent developments in the understanding of the structure and properties of grain boundaries in metals

The basic physical problems of the geometric models and structural models of grain boundaries based on computer simulations are considered. By comparing the approximations made in these models with the general solution of the problem based on the theory of metallic cohesion, we are led to conclude that

• i geometric grain boundary models (coincidence models) may be used to derive certain geometric features of a boundary (e.g. the boundary periodicity). However, due to the fact that interatomic forces are neglected, it seems not possible to deduce the actual atomic structure and properties of grain boundaries on the basis of these models.
• ii Dislocation models of grain boundaries seem to be of physical significance only for low energy boundaries. In all other boundaries, misfit dislocation are theoretically and experimentally found to be delocalized in the sense that their core is smeared out in the plane of the boundary.
• iii Structural boundary models deduced by computing the minimum energy atomic configuration using pairwise interatomic potential functions seem to represent a reasonable approximation of the actual atomic structure.
• iv The experimental and theoretical evidence presented indicates that grain boundary properties can, in general, not be derived from computer models, as these properties depend partially on electronic effects.
• v Further developments of the physical understanding of grain boundaries seems to require the incorporation of electronic effects in the theory of interfaces.