压力下光学声子对应变纤锌矿AlN/GaN异质结中电子迁移率的影响

A variational method combined with solving the force balance equation is adopted to investigate the influence of strain and hydrostatic pressure on electronic mobility in a strained wurtzite AlN/GaN heterojunction by considering the scattering of optical-phonons in a temperature ranges from 250 to 600 K. The effects of conduction band bending and an interface barrier are also considered in our calculation. The results show that electronic mobility decreases with increasing hydrostatic pressure when the electronic density varies from 1.0 × 1012 to 6.5 × 1012 cm-2. The strain at the heterojunction interface also reduces the electronic mobility, whereas the pressure influence becomes weaker when strain is taken into account. The effect of strain and pressure becomes more obvious as temperature increases. The mobility first increases and then decreases significantly, whereas the strain and hydrostatic pressure reduce this trend as the electronic density increases at a given temperature （300 K）. The results also indicate that scattering from half space phonon modes in the channel side plays a dominant role in mobility.     

改善SiGe HBTs热稳定性的Ge组分分布优化与设计

The impact of the three state-of-the-art germanium(Ge) profiles(box,trapezoid and triangular) across the base of SiGe heterojunction bipolar transistors(HBTs) under the condition of the same total amount of Ge on the temperature dependence of current gainβand cut-off frequency f_T,as well as the temperature profile,are investigated.It can be found that although theβof HBT with a box Ge profile is larger than that of the others,it decreases the fastest as the temperature increases,while theβof HBT with a triangular Ge profile is smaller than that of the others,but decreases the slowest as the temperature increases.On the other hand,the f_T of HBT with a trapezoid Ge profile is larger than that of the others,but decreases the fastest as the temperature increases,and the f_T of HBT with a box Ge profile is smaller than that of the others,but decreases the slowest as temperature increases.Furthermore,the peak and surface temperature difference between the emitter fingers of the HBT with a triangular Ge profile is higher than that of the others.Based on these results,a novel segmented step box Ge profile is proposed,which has modestβand f_T,and trades off the temperature sensitivity of current gain and cut-off frequency,and the temperature profile of the device.     

Characterization of Interface Strength as Function of Temperature and Moisture Conditions

Since Moisture Sensitivity Level (MSL) tests are part of the international reliability qualification standards, all the microelectronics components/products have to pass these specifications. Therefore, it is important to be able to efficiently and accurately characterize and predict the moisture related material and interface behavior in the real manufacturing, processing, testing and application conditions. The success of interfacial fracture mechanics approach to analyze moisture-induced failures in IC packaging strongly depend on accurate characterization of the critical adhesion strength, Gc. However, its measurement is complicated by the fact that adhesion depends not only on moisture concentration, C, but also temperature, T, and mode mixity, ψ. This paper described our research to develop a reliable methodology for interface toughness evaluation as function of temperature, humidity and mode mixity. Our methodology includes using the four-point bending test and shaft-loaded-blister method. Dedicated specimens consisting of various types of moulding compounds bonded onto leadframe are manufactured. Besides temperature, moisture content and mode mixity effects, also the influences of surface treatment (leadframe oxidation and contamination) and production process on the interface fracture toughness are evaluated. Multi-physics-based numerical methods are used to transfer the experimental critical loads to an interface strength parameter. These analysis covers mechanical, moisture diffusion, vapor pressure, hygro-swelling and CTE-mismatch modeling. To test and improve the methodology, various effects are evaluated, such as crack-length dependency, material properties, specimen- width, displacement-rate of the upper support/shaft, etc. The results of the proposed methodology indicate, as expected, a change in interface toughness by mode mixity, moisture content and temperature. It is found that Gc decreases with increasing moisture content and temperature. The presence of moisture at the given interface is observed as the important factor in the reduction of interfacial strength (>>20 %～45%). Furthermore, Gc increases by a factor 3～4 when the mode mixity shifts towards mode II.     

Structural, morphological, dielectrical and magnetic properties of Mn substituted cobalt ferrite

The Co1 xMnxFe2O4(06x60.5) ferrite system is synthesized by using an auto combustion technique using metal nitrates. The influence of Mn substitution on the structural, electrical, impedance and magnetic properties of cobalt ferrite is reported. X-ray diffraction patterns of the prepared samples confirm that the Bragg’s peak belongs to a spinel cubic crystal structure. The lattice constant of cobalt ferrite increases with the increase in Mn content. The microstructural study is carried out by using the SEM technique and the average grain size continues to increase with increasing manganese content. AC conductivity analysis suggests that the conduction is due to small polaron hopping. DC electrical resistivity decreases with increasing temperature for a Co1 xMnxFe2O4system showing semiconducting behavior. The activation energy is found to be higher in the paramagnetic region than the ferromagnetic region. Curie temperature decreases with Mn substitution in the host ferrite system. Dielectric dispersionhavingMaxwell-Wagner-typeinterfacialpolarizationhasbeenobservedforcobaltferritesamples.Magnetic properties have been studied by measuring M-H plots. The saturation and remanent magnetization increases with Mn substitution.     

Effects of p-type GaN thickness on optical properties of GaN-based light-emitting diodes

The influence of p-type Ga N(p Ga N) thickness on the light output power(LOP) and internal quantum efficiency(IQE) of light emitting diode(LED) was studied by experiments and simulations. The LOP of Ga N-based LED increases as the thickness of p Ga N layer decreases from 300 nm to 100 nm, and then decreases as the thickness decreases to 50 nm. The LOP of LED with 100-nm-thick pG a N increases by 30.9% compared with that of the conventional LED with 300-nm-thick p Ga N. The variation trend of IQE is similar to that of LOP as the decrease of Ga N thickness. The simulation results demonstrate that the higher light efficiency of LED with 100-nm-thick p Ga N is ascribed to the improvements of the carrier concentrations and recombination rates.     

DETECTION PERFORMANCE OF DIGITAL POLARITY SAMPLED PHASE REVERSAL CODED PULSE COMPRESSORS

The nonparametic constant false alarm rate(CFAR)property of digital polarity sampled phasereversal coded pulse compressors is described.The detection performance in Gaussian and non-Gaussiannoise is determined.It is shown that the loss in signal-to-noise ratio of the processor relative to the incoherentmatched filter decreases as the code length increases.The asymptotic loss in Gaussian noise is 1.96 dB,and theloss in Weibull noise decreases with the shape parameter of the Weibull distribution and can even become again.     

Effects of hydrostatic pressure and external electric field on the impurity binding energy in strained GaN/AlxGa1?xN spherical quantum dots

The binding energy and Stark effect energy shifts of a shallow donor impurity state in a strained GaN/AlxGa1-xN spherical finite-potential quantum dot (QD) are calculated using a variational method based on the effective mass approximation. The binding energy is computed as a function of dot size and hydrostatic pressure. The numerical results show that the binding energy of the impurity state increases, attains a maximum value, and then decreases as the QD radius increases for any electric field. Moreover, the binding energy increases with the pressure for any size of dot. The Stark shift of the impurity energy for large dot size is much larger than that for the small dot size, and it is enhanced by the increase of electric field. We compare the binding energy of impurity state with and without strain effects, and the results show that the strain effects enhance the impurity binding energy considerably, especially for the small QD size. We also take the dielectric mismatch into account in our work.     

THE DIFFUSION OF Zn IN GaAs AT LOW TEMPERATURE

The diffusion of Zn into GaAs at low temperature has been investigated.The experiments arecarried out in an evacuated and sealed quartz ampoule using ZnAs_2 as the source of Zn.The relation among the junction depth(X_j),the time(t)and the temperature(T)of diffusion hasbeen investigated.It is found that the sheet resistance(R_s)of diffusion layer increases as X_j decreases.The surface concentration(C_s)decreases as 1/T increases,and mobility(μ)decreases as C_s increasesThe C_s versus 1/(X_j,R_s)are plotted,the results are that C_s increases as 1/(X_j,R_s)increases.This is asimple method for determining C_s of the multiple GaAs/GaAlAs epitaxial layer.The mechanism ofZn diffusion in GaAs and InP is discussed.This process has been applied to fabricate GaAs/GaAlAsdouble heterojunction light emitting diodes and an output power of 2—4mW is obtained,the seriesresistance is 3—5Ω.     

Analysis of Vertical Oscillations of a Permanent Magnet Freely Levitated above an YBCO Bulk in an AC External Magnetic Field

The effect of the moving speed of permanent magnet （PM） on levitation force between PM and high temperature superconducting （HTS） bulk is analyzed and described in the PM-HTS levitation system. The PM vibration characteristic in the PM-HTS system is investigated. The PM may collide with the HTS in vibration if the amplitude and frequency of driving force satisfy the relationship Pmin=Afn. When the load of the system is below a threshold, the minimal collision amplitude of the driving force increases with the load increasing, however, it sharply drops to zero when the load exceeds the threshold. With the increase of the initial height of the PM, the threshold load increases, but the minimal driving force which causes a collision between PM and HTS decreases.     

Host load prediction in cloud based on classification methods

The host load prediction problem in cloud computing has also been received much attention. To solve this problem, we have to use the historical load data to predict the future load level. Accurate prediction methods are useful for host load balance and virtual machine migration. Although cloud is likely to grids at some extent, the length of tasks are much shorter and host loads change more frequently with higher noise. The above characteristics introduce challenges for host load prediction. In this paper, based on the proposed exponentially segmented pattern and the corresponding transformation, prediction problem is transformed into the traditional classification problem, This classification problem can be solved based on the traditional methods, and features are given for training the classification model. For achieving accurate prediction, a new feature periodical coefficient is introduced and some existed classification methods are implemented. Experiments on the real world dataset invalidate the efficiency of the new proposed feature, which is in the most effective combinations of features, it increases successful rate （SR） 1.33%-2.82% and decreases the mean square error （MSE） 1.37%-2.91%. And the results also show that support vector machine （SVM） method can achieve nearly the same performance as the Bayes methods and their performance is about 50% higher in successful rate and 17% better in the mean square error compared to the existed methods.     

Atomistic simulations of the tensile and melting behavior of silicon nanowires

Molecular dynamics simulations with Stillinger-Weber potential are used to study the tensile and melting behavior of single-crystalline silicon nanowires (SiNWs). The tensile tests show that the tensile behavior of the SiNWs is strongly dependent on the simulation temperature, the strain rate, and the diameter of the nanowires.For a given diameter, the critical load significantly decreases as the temperature increases and also as the strain rate decreases. Additionally, the critical load increases as the diameter increases. Moreover, the melting tests demonstrate that both melting temperature and melting heat of the SiNWs decrease with decreasing diameter and length, due to the increase in surface energy. The melting process of SiNWs with increasing temperature is also investigated.     

TEM characterization of Si nanowires grown by CVD on Si pre-structured by nanosphere lithography

Silicon nanowires (SiNWs) were grown on Si(1 0 0) and Si(1 1 1) substrates by chemical vapour deposition (CVD) via the vapour–liquid–solid (VLS) mechanism with small gold particles used as seeds. In order to control the diameter of nanowires, their density on the substrate and their orientation we controlled the size and the distribution of Au seed particles. This was accomplished using nanosphere lithography (NSL) by which regular arrays of Au nanoparticles can be generated. This allowed us to grow single-crystalline SiNWs perpendicular to the surface of Si(1 1 1) substrates. The SiNWs and their Au caps were studied with respect to their morphology and composition using TEM, HREM and EFTEM methods. Clusters of Au are observed along the surface of SiNWs and the existence of a thin Si film on gold particles capping the SiNWs is demonstrated.     

Vertically-tapered silicon nanowire arrays prepared by plasma enhanced chemical vapor deposition: Synthesis,structural characterization and photoluminescence

Vertically aligned silicon nanowires (SiNWs) have been successfully synthesized using pure silane gas as a precursor by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) method. The effect of the growth temperature on the morphology, structure and photoluminescence properties of SiNWs has been studied. The SiNWs were needle-liked materials with the length of a few microns having the diameters of tens of nanometers near the bottom and a few nanometers at the top. Thinner nanowires have been obtained at the higher growth temperature process. High resolution transmission electron microscopy confirms that the nanowires are composed of a crystalline silicon core with an oxide shell. The PL spectrum of the Si nanoneedles have shown two emission bands around 450 nm and ~750, which originate from the defects related to oxygen fault in the oxide shell and interfaces between the crystalline Si core and the oxide shell, respectively.     

Study of the Piezoelectric Power Generation of ZnO Nanowire Arrays Grown by Different Methods

The piezoelectric power generation from ZnO nanowire arrays grown on different substrates using different methods is investigated. ZnO nanowires were grown on n‐SiC and n‐Si substrates using both the high‐temperature vapor liquid solid (VLS) and the low‐temperature aqueous chemical growth (ACG) methods. A conductive atomic force microscope (AFM) is used in contact mode to deflect the ZnO nanowire arrays. No substrate effect was observed but the growth method, crystal quality, density, length, and diameter (aspect ratio) of the nanowires are found to affect the piezoelectric behavior. During the AFM scanning in contact mode without biasing voltage, the ZnO nanowire arrays grown by the VLS method produced higher and larger output voltage signal of 35 mV compared to those grown by the ACG method, which produce smaller output voltage signal of only 5 mV. The finite element (FE) method was used to investigate the output voltage for different aspect ratio of the ZnO nanowires. From the FE results it was found that the output voltage increases as the aspect ratio increases and starts to decreases above an aspect ratio of 80 for ZnO nanowires.     

A systematic study of the impact of etching time to the sensitivity of SiNW sensor fabricated by MACEtch process

In this paper, silicon nanowires (SiNWs) was fabricated by a combination of metal-assisted chemical etching (MACEtch) and nanosphere lithography. We get the silicon nanowires with different specific surface area by changing the etching time. The microscopic structure of the silicon nanowires is observed by field emission scanning electron microscope (FESEM). The gas sensing performances of the SiNWs with different specific surface area have been systematically examined by measuring the resistance change towards the concentrations of NO₂ in the range of 1–5 ppm at room temperature (RT, 300 K), the gas sensor composed of SiNWs showed perfect gas sensitive property and possessed a short response–recovery time. The main reason of these excellent attributes is quite likely that high specific surface area of the SiNWs, and NO₂ sensing mechanism of the SiNWs was also further explained, which can be attributed to the oxygen in the air and detected NO₂ extract electrons from the surface of the SiNWs, and the resistivity of SiNWs changed with the changing of space-charge layer under the of SiNWs surface.     

A New Route to Large‐Scale Synthesis of Silicon Nanowires in Ultrahigh Vacuum

An approach for the large‐scale synthesis of high‐purity silicon nanowires (SiNWs) in ultrahigh vacuum is presented. A mixture of Si and SiO₂ is evaporated by an electron beam, and the growth temperature is 700 °C, which is much lower than those used for other oxide‐assisted growths. A new type of single‐crystal SiNWs, with [221] orientation, is thus synthesized. Moreover, it is experimentally demonstrated that SiO intermediates are formed in the process, and the nanowires are obtained via a disproportionation reaction of 2SiO → Si + SiO₂. A growth mechanism is proposed and the critical factors for the formation of 1D nanowires are also determined. The approach is particularly compatible with the mature Si‐based technology, and is favorable for device integration and practical applications.     

Structural and optical properties of silicon nanowires synthesized by Ag-assisted chemical etching

Metal-assisted chemical etching (MACE) of silicon in an aqueous solution of hydrofluoric acid and hydrogen peroxide is established for the fabrication of large-area uniform silicon nanowire (SiNW) arrays. The effect of the silver catalyst layer thickness on the morphology of the synthesized nanostructures and nanowires is investigated. Atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM) reveal that the morphology of the fabricated silicon nanostructures remarkably depends on the catalyst layer thickness, and an optimum layer thickness is necessary for the fabrication of SiNWs. Also the effect of different etching times on the structural and optical properties of the fabricated SiNWs is investigated. FESEM showed a linear increment of the nanowire length and slight diameter changes through different etching times. The ultralow reflectance of SiNWs in the absorption region through the measurement of specular and diffuse reflectance showed that with increase in the etching time, the total reflectance remarkably decreases. A broadband visible photoluminescence (PL) emission from these wires was observed, and it could be stated that the silicon nanocrystals (SiNCs) are mostly responsible for the PL emission. The SiNC sizes were determined by an analytical model through a frequency shift in the Raman spectrum. The synthesized optically-active SiNWs could, therefore, be considered as a promising candidate for a new generation of nanoscale opto-electronic devices.     

Formation of Luminescent Silicon Nanowires and Porous Silicon by Metal-Assisted Electroless Etching

Metal-assisted etching of silicon in HF/H₂O₂ aqueous solutions has been used to fabricate luminescent silicon nanowires (SiNWs) and porous silicon. The impact of the gold catalyst layer thickness and the etching solution on the morphology of the synthesized nanostructures and the diameter of the obtained nanowires were systematically investigated. Scanning electron microscopy (SEM) analyses reveal that the morphology of the fabricated structures strongly depends on the composition of the solution and the thickness of the catalyst layer. It has been observed that SiNWs are formed in solutions with H₂O₂ ratios (ξ) below 10 %; increasing the H₂O₂ concentration above this critical value leads to mesoporous (10 % < ξ < 14 %) and macroporous (14 % < ξ < 17 %) structures. Photoluminescence measurements show that SiNWs emit light at about 430 nm. Fourier transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM) analyses were utilized to determine the origin of the emission in the silicon nanostructures. TEM imaging demonstrates that SiNWs are covered by a thin layer of porous silicon, which is assumed to be responsible for their light emission.     

Effects of dopant,temperature, and strain rate on the mechanical properties of micrometer gold-bonding wire

An experimental study on the thermomechanical properties of microelectronic gold-bonding wires was conducted using high-precision, microforce tensile tests. The load-displacement behavior of three types of gold wire (GL-2 type, FA type, and SR type) was carefully recorded for determining the elastic modulus and stress-strain curves. Tests were conducted with miniature specimens at temperatures ranging from room temperature to 250°C and load rates on the order of 1 mm/min and 10 mm/min. The testing results indicated that the tensile strength of all three types of wire decreased with temperature, especially the SR type. The load strain rate has a significant effect on the SR type but little effect on the GL-2 and FA wires. The stress-strain curves from these tests were analyzed to fit into empirical constitutive models that account for the strain, temperature, and strain-rate effects.     

Growth of silicon nanowires on H-terminated Si {111} surface templates studied by transmission electron microscopy

We have studied the growth of silicon nanowires (SiNWs) by means of transmission electron microscopy. SiNWs are grown from nanocatalysts via the Vapor-Liquid-Solid (VLS) mechanism using silane (SiH4) gas as a source gas. The nanocatalysts are prepared on a hydrogen (H)-terminated Si surface. We have examined the formation mechanism of nanocatalysts on H-terminated surface and have observed several structural variants of SiNWs. According to the study we have suggested that many structural variations of SiNWs are possible, which modify the structural properties of SiNWs to great extents.