Impact of [110]/(001) uniaxial stress on valence band structure and hole effective mass of silicon

The valence band structure and hole effective mass of silicon under a uniaxial stress in(001) surface along the[110]direction were detailedly investigated in the framework of the k·p theory.The results demonstrated that the splitting energy between the top band and the second band for uniaxial compressive stress is bigger than that of the tensile one at the same stress magnitude,and of all common used crystallographic direction,such as[110], [001],[110]and[100],the effective mass for the top band along[110]crystallographic direction is lower under uniaxial compressive stress compared with other stresses and crystallographic directions configurations.In view of suppressing the scattering and reducing the effective mass,the[110]crystallographic direction is most favorable to be used as transport direction of the charge carrier to enhancement mobility when a uniaxial compressive stress along[110]direction is applied.The obtained results can provide a theory reference for the design and the selective of optimum stress and crystallorgraphic direction configuration of uniaxial strained silicon devices.     

A Homotopy—Controlled Mean Field Annealing Partially—Connected Neural Equalizer for Pan—European GSM Syst5em

Channel equalization is essential in the Pan-European GSM mobile communication system.The maximum likelihood sequence estimation(MLSE) using the Viterbi algorithm(VA)iscommonly recommended for the dqualization,which can only accommodate the channels with limited time delay spread.In[1],we presented a mean field annealing(MFA)partially connected neural equalizer for the GSM system,in which the complexity is linearly proportional to the time delay spread and therefore relatively fast convergence speed is achieved.But the annealing coefficient of the MFA equalizer is fixed,which is not flexible in timing-varying circumstance such as mobile communications.To decrease the computation of MFA approach so as to make it more easy for practical use,the MFA approach is reated as a homotopy problem.The ordinary equations which the MFA approach should obey are derived.These equations can be used to reflect the deviation of the iteration result from the track of MFA approach.Based on this tesult,an adaptive annealing control algorithm is proposed,which can dynamically control the annealing coefficient according to the iteration deviation.Computer simulations show that our approach can provide a much higher convergence speed and performance improvement over 16-state and 32-state VA‘s which are usually suggested for practical applications.     

Metal dopants in HfO2-based RRAM: first principle study

Based on density-functional theory （DFT）, the effects of metal dopants in HfO2-based RRAM are studied by the Vienna ab initio simulation package （VASP）, Metal dopants are classified into two types （interstitial and substitutional） according to the formation energy when they exist in HfO2 cell. Several conductive channels are observed through the isosurface plots of the partial charge density for HfO2 doped with interstitial metals, while this phenomenon cannot be found in HfO2 doped with substitutional metals. The electron density of states （DOS） and the projected electron density of states （PDOS） are calculated and analyzed; it is found that the conduction filament in HfO2 is directly formed by the interstitial metals and further, that the substitutional metals cannot directly generate conduction filament. However, all the metal dopants contribute to the formation of the oxygen vacancy （Vo） filament. The formation energy of the Vo and the interaction between metal dopants and Vo are calculated; it is revealed that the P-type substitutional metal dopants have a strong enhanced effect on the Vo filament, the interstitial metal dopants have a minor assistant effect, while Hf-like and N-Woe substitutional metal dopants have the weakest assistant effect.     

Computation and analysis of aero-optic imaging deviation of a blunt nosed aircraft with Mach number 0.5—3

Aero-optic imaging is a kind of optical effect, which describes the imaging deviation on the imaging plane. In this paper, the effect of the change of Mach number of blunt aircraft on the aero-optic imaging deviation is studied. The imaging deviations of Mach number 0.5—3 are analyzed systematically. The results show that with the increase of Mach number, imaging deviation increases gradually, and the increase rate is gradually slow. Imaging deviation slope decreases gradually with the increase of Mach number, and gradually tends to be zero, suggesting that imaging deviation is not sensitive to the change of the larger Mach number. In other words, the Mach number of smaller changes can lead to larger imaging deviation. As the Mach number of the aircraft increases, the slope of the imaging offset tends to be closer and closer to 0. When the Mach number of the aircraft increases to a certain extent, the change of the imaging offset will not have much influence. Therefore, in order to reduce the impact of flight speed on imaging migration, the aircraft should fly at a higher Mach number.     

对高速比较器中复位场效应管的研究

A high-speed comparator design based on regeneration architecture, which can be used in a flash ADC, is presented.A threshold-limit-speed effect（TLSE） which limits the speed of the comparator was discovered and studied in detail.The size of the reset-MOSFET was optimized to resolve the TLSE and make the comparator work at the maximal speed.The results were confirmed by simulation and the corresponding circuit was realized in a flash ADC design in SMIC 0.18-μm CMOS technology.The test result shows that the comparator can work well at 2 GHz and can even work up to 2.8 GHz while the power dissipation is 3.2 mW.     

Structural and optical properties of polycrystalline CdS thin films deposited by electron beam evaporation

Highly crystalline and transparent cadmium sulphide(CdS) films were deposited on glass substrate by electron beam evaporation technique.The structural and optical properties of the films were investigated.The X-ray diffraction analysis revealed that the CdS films have a hexagonal structure and exhibit preferred orientation along the(002) plane.Meanwhile,the crystalline quality of samples increased first and then decreased as the substrate temperature improved,which is attributed to the variation in film thickness.UV-vis spectra of CdS films indicate that the absorption edge becomes steeper and the band gap present fluctuation changes in the range of 2.389-2.448 eV as the substrate temperature increased.The photoluminescence peak of the CdS films was found to be broadened seriously and there only emerges a red emission band at 1.60 eV.The above results were analyzed and discussed.     

Design of a high performance CMOS charge pump for phase-locked loop synthesizers

A new high performance charge pump circuit is designed and realized in 0.18μm CMOS process. A wide input ranged rail-to-rail operational amplifier and self-biasing cascode current mirror are used to enable the charge pump current to be well matched in a wide output voltage range.Furthermore,a method of adding a precharging current source is proposed to increase the initial charge current,which will speed up the settling time of CPPLLs.Test results show that the current mismatching can be less than 0.4%in the output voltage range of 0.4 to 1.7 V,with a charge pump current of 100μA and a precharging current of 70μA.The average power consumption of the charge pump in the locked condition is around 0.9 mW under a 1.8 V supply voltage.     

缺陷相互作用对HfO2基阻变存储器件均匀性的影响：第一性原理研究

The physical mechanism of doping effects on switching uniformity and operation voltage in Al-doped HfO₂ resistive random access memory（RRAM） devices is proposed from another perspective:defects interactions, based on first principle calculations.In doped HfO₂,dopant is proved to have a localized effect on the formation of defects and the interactions between them.In addition,both effects cause oxygen vacancies（V_O） to have a tendency to form clusters and these clusters are easy to form around the dopant.It is proved that this process can improve the performance of material through projected density of states（PDOS） analysis.For V_O filament-type RRAM devices, these clusters are concluded to be helpful for the controllability of the switching process in which oxygen vacancy filaments form and break.Therefore,improved uniformity and operation voltage of Al-doped HfjO₂ RRAM devices is achieved.     

表面结构和生长方向对WO3纳米线电子结构影响的第一性原理研究

The effects of the surface and orientation of a WO₃ nanowire on the electronic structure are investigated by using first principles calculation based on density functional theory（DFT）.The surface of the WO3 nanowire was terminated by a bare or hydrogenated oxygen monolayer or bare WO₂ plane,and the[010]- and[001]-oriented nanowires with different sizes were introduced into the theoretical calculation to further study the dependence of electronic band structure on the wire size and orientation.The calculated results reveal that the surface structure, wire size and orientation have significant effects on the electronic band structure,bandgap,and density of states （DOS） of the WO₃ nanowire.The optimized WO₃ nanowire with different surface structures showed a markedly dissimilar band structure due to the different electronic states near the Fermi level,and the O-terminated[001] WO₃ nanowire with hydrogenation can exhibit a reasonable indirect bandgap of 2.340 eV due to the quantum confinement effect,which is 0.257 eV wider than bulk WO₃.Besides,the bandgap change is also related to the orientation-resulted surface reconstructed structure as well as wire size.     

Cadmium sulfide thin films growth by chemical bath deposition

Cadmium sulfide (CdS) thin films have been prepared by a simple technique such as chemical bath deposition (CBD).A set of samples CdS were deposited on glass substrates by varying the bath temperature from 55 to 75 ℃ at fixed deposition time (25 min) in order to investigate the effect of deposition temperature on CdS films physical properties.The determination of growth activation energy suggests that at low temperature CdS film growth is governed by the release of Cd2+ ions in the solution.The structural characterization indicated that the CdS films structure is cubic or hexagonal with preferential orientation along the direction (111) or (002),respectively.The optical characterization indicated that the films have a fairly high transparency,which varies between 55％ and 80％ in the visible range of the optical spectrum,the refractive index varies from 1.85 to 2.5 and the optical gap value of which can reach 2.2 eV.It can be suggested that these properties make these films perfectly suitable for their use as window film in thin films based solar cells.     

Compliance current dominates evolution of NiSi2 defect size in Ni/dielectric/Si RRAM devices

Resistive random access memory (RRAM) devices with a nickel top electrode form controllable metal nanofilaments and have robust resistive switching performance. We investigate the Ni/HfO₂/SiO_x/n⁺ Si RRAM structure, which forms a Ni-rich defect in the silicon underneath the Ni nanofilament in the dielectric layers after a SET process. The formation of these defects may affect the retention of the devices, so we applied a detailed Finite Element Method and Kinetic Monte Carlo approach to simulate the Ni-rich defect evolution under different compliance current settings. We confirm that the chemical composition of the defects is metallic NiSi₂, and that their size is determined by the compliance current. These simulation results are supported by in-situ STM-like experiments inside a transmission electron microscope (TEM). NiSi₂ defects are shaped as truncated square pyramids, and we show that this is due to the low activation energy of Ni migration along the (111) crystal plane of Si. Our results demonstrate that electromigration is the main driving force for Ni migration initially, after which thermal migration and especially stress migration become the dominant mechanism. This work gives a fascinating example of an as-grown metal–insulator–semiconductor (MIS) system that can be controllably converted to a metal–insulator–metal (MIM) configuration for down-scaled RRAM operation.     

Defect formation in epitaxial oxide dielectric layers due to substrate surface relief

Defects were characterized in epitaxial (001) CeO₂ films deposited and planarizedin situ on patterned (001) LaAlO₃ substrates by ion beam assisted deposition (IBAD). A hill and valley structure with steps running parallel to the [100] LaAlO₃ axis was produced on the surface of the substrate by photolithography and ion beam etching prior to film deposition. A conformai epitaxial CeO₂ layer of ∼ 100 nm thickness was deposited on the heated substrate by e-beam evaporation. Lattice-matching between the e-beam film and the substrate was of the type: (001) CeO₂∥(001) LaAlO₃ and [110] CeO₂∥[100] LaAlO₃. Evaporative deposition of additional film onto the conformai layer was accompanied by bombardment with a 500 eV argon/oxygen ion beam to promotein situ planarization. Extreme lattice misfit for the orientation (001) CeO₂∥(001)LaAlO₃ and [001] CeO₂∥[001] LaAlO₃ caused formation of dislocations in the e-beam CeO₂ film in the vicinity of individual ledges in the substrate surface. Coherence of the CeO₂ film was locally lost in the step regions of the hill and valley structure. The large patterned steps, which are composed of numerous adjacent ledges in the LaAlO₃ surface, caused nucleation of CeO₂ with a tilt misalignment of up to ∼5‡ about the substrate [100]. Nucleation and growth of nonepitaxial CeO₂ crystallites was observed along the step regions of the film during the IBAD portion of deposition. Defect formation in the e-beam ceria layer due to substrate surface relief indicates that “lattice engineering≓ of multilayer epitaxial structures may not be possible when nonplanar surfaces are created during device fabrication. The IBAD CeO₂ layer was more defective than the conformai layer deposited without the impinging ion beam, even in the portions of the film where epitaxy was maintained throughout both layers.     

Realization of the Switching Mechanism in Resistance Random Access Memory™ Devices: Structural and Electronic Properties Affecting Electron Conductivity in a Hafnium Oxide–Electrode System Through First-Principles Calculations

The resistance random access memory (RRAM?) device, with its electrically induced nanoscale resistive switching capacity, has attracted considerable attention as a future nonvolatile memory device. Here, we propose a mechanism of switching based on an oxygen vacancy migration-driven change in the electronic properties of the transition-metal oxide film stimulated by set pulse voltages. We used density functional theory-based calculations to account for the effect of oxygen vacancies and their migration on the electronic properties of HfO₂ and Ta/HfO₂ systems, thereby providing a complete explanation of the RRAM? switching mechanism. Furthermore, computational results on the activation energy barrier for oxygen vacancy migration were found to be consistent with the set and reset pulse voltage obtained from experiments. Understanding this mechanism will be beneficial to effectively realizing the materials design in these devices.     

铪/二氧化铪基双极阻变存储器特性研究

本文制备了纳米级Hf/HfO2阻变存储器（RRAM）。RRAM顶层电极和底部电极交叉，从而形成了金属-氧化物-金属结构。系统地研究了RRAM的电学特性，包括forming过程，SET过程和RESET过程。讨论了SET电压和RESET电压的相关性，以及高阻态和低阻态的相关性。RRAM的电学特性与SET过程中的限制电路强相关。可以基于量子点接触模型，阐述纳米级Hf/HfO2阻变存储器的导通机制。     

High κ for MIM and RRAM applications: Impact of the metallic electrode and oxygen vacancies

Influence of metallic electrode and oxygen vacancies in MIM capacitors and MIM RRAM high κ based devices is studied. For both MIM capacitors and MIM RRAM it is shown that the electrode composition strongly influences the overall behavior of the devices and more precisely, the capacitance–voltage curve (nonlinearities) for MIM capacitors, and the switching mechanism (SET/RESET) for MIM RRAM. Best results for HfO₂ RRAM are obtained with Pt as bottom electrode instead of TiN while very low capacitance variations are observed for high work function electrodes, or more precisely electrodes with low oxygen affinity. These evolutions are related to the oxygen vacancies concentration and migration to the cathode electrode/high κ interface.     

Study of radiation hardness of HfO2-based resistive switching memory at nanoscale by conductive atomic force microscopy

A resistive switching random access memory (RRAM) with an HfO₂/Ti structure grown on a molybdenum (MO) substrate was fabricated, and a gold (Au) conductive atomic force microscopy (CAFM) tip was used as the top electrode such that the cell area of the resulting RRAM device is as small as 3 × 10⁻¹² cm². The pre- and post-irradiated resistive switching behaviors of the RRAM device with various HfO₂ layer thicknesses were investigated after being subjected to Co⁶⁰ γ-ray irradiation with different radiation doses. It is found that the forming voltage (V_forming), set voltage (V_set), resistance of high resistance state (RHRS) and resistance of low resistance state (RLRS) of the RRAM device are all radiation dose-dependent. The V_forming, V_set, RHRS and RLRS all decrease as the radiation dose increases due to increasing radiation-induced oxygen vacancies or defects inside the HfO₂ layer. Our experimental results indicate that the HfO₂-based RRAM cell with an extremely small cell area is not actually radiation hard since the operating voltage will change with V_forming and V_set after irradiation.     

Suppression of twin formation in CdTe(111)B epilayers grown by molecular beam epitaxy on misoriented Si(001)

CdTe(lll)B layers have been grown on misoriented Si(001). Twin formation inside CdTe(lll)B layer is very sensitive to the substrate tilt direction. When Si(001) is tilted toward [110] or [100], a fully twinned layer is obtained. When Si(001) is tilted toward a direction significantly away from [110], a twin-free layer is obtained. Microtwins inside the CdTe(111)B layers are overwhelmingly dominated by the lamellar twins. CdTe(111)B layers always start with heavily lamellar twinning. For twin-free layers, the lamellar twins are gradually suppressed and give way to twin-free CdTe(111)B layer. The major driving forces for suppressing the lamellar twinning are the preferential orientation of CdTe[11-2] along Si[1-10] and lattice relaxation. Such preferential orientation is found to exist for the CdTe(111)B layers grown on Si(001) tilted toward a direction between [110] and [100].     

The one-step vacuum growth of high-quality CuInS2 thin film suitable for photovoltaic applications

Due to its direct energy band gap of 1.53 eV, which is well matched to the solar spectrum, the ternary compound CuInS₂ becomes a promising absorber material for high conversion efficiency solar cells. We report in this paper the preparation and characterization of improved quality CuInS₂ films for use as a high-efficiency solar cell absorber. The films were deposited by RF reactive sputter technique, in which the Cu–In alloy target, H₂S reactant gas, and soda lime glass and Si wafer substrates were used. The as-deposited films were the CuInS₂ chalcopyrite single phase with the preferred orientation of (1 1 2). Void-free films with a grain size of about 400 nm and the constituent ratio [Cu+In]/[S] and [Cu]/[Cu+In] approaching 1 and 0.5, respectively, could be attained by optimizing the process parameters, and films with outstanding electrical characteristics could thus be obtained.