Investigation of defect structure of neutron-irradiated crystalline silicon by low-temperature thermal conductivity

The thermal conductivity of initially n-type silicon irradiated at temperatures higher than room temperature with fast neutron doses between 2·10¹⁸ and 2·10¹⁹ n/cm² and following thermal annealing at 375 °C and 800 °C was measured at low temperatures (2–200 K). Additionally, the heat capacity was investigated in the temperature range from 2 K to 50 K. With the help of the analysis of (T) on the basis of the Callaway model, results on the nature of neutron-induced defects and their change by annealing were obtained. The neutron-induced damage can be described by point defects and by extended defects in the kind of dislocations or dislocation loops, respectively. The interaction of point defects with phonons appears as Rayleigh scattering and partially as resonance-like scattering. At the Si sample with highest neutron dose, an anomalous behavior (cooling effect) was observed after annealing at 375 °C. The state of structure is not stable by cooling between room temperature and 78 K and therefore a decrese of (T) with each cooling cycle is obtained. A significant change of specific heat was not observed by neutron irradiation.     

Ultralow thermal conductivity of isotope-doped silicon nanowires

The thermal conductivity of silicon nanowires (SiNWs) is investigated by molecular dynamics (MD) simulation. It is found that the thermal conductivity of SiNWs can be reduced exponentially by isotopic defects at room temperature. The thermal conductivity reaches the minimum, which is about 27% of that of pure 28Si NW, when doped with 50% isotope atoms. The thermal conductivity of isotopic-superlattice structured SiNWs depends clearly on the period of superlattice. At a critical period of 1.09 nm, the thermal conductivity is only 25% of the value of pure Si NW. An anomalous enhancement of thermal conductivity is observed when the superlattice period is smaller than this critical length. The ultralow thermal conductivity of superlattice structured SiNWs is explained with phonon spectrum theory.     

Electrical conductivity of single crystal nickel tungstate

Measurements of a.c. and d.c. electrical conductivity in crystals of nickel tungstate, in the temperature range 300 to 1100 K, are presented. NiWO₄ is found to be a semiconductor with a band gap of 2.10 eV. The nature of the electrical conduction is discussed in the light of various conduction models.     

Unveiling the formation pathway of single crystalline porous silicon nanowires

Porous silicon nanowire is emerging as an interesting material system due to its unique combination of structural, chemical, electronic, and optical properties. To fully understand their formation mechanism is of great importance for controlling the fundamental physical properties and enabling potential applications. Here we present a systematic study to elucidate the mechanism responsible for the formation of porous silicon nanowires in a two-step silver-assisted electroless chemical etching method. It is shown that silicon nanowire arrays with various porosities can be prepared by varying multiple experimental parameters such as the resistivity of the starting silicon wafer, the concentration of oxidant (H(2)O(2)) and the amount of silver catalyst. Our study shows a consistent trend that the porosity increases with the increasing wafer conductivity (dopant concentration) and oxidant (H(2)O(2)) concentration. We further demonstrate that silver ions, formed by the oxidation of silver, can diffuse upwards and renucleate on the sidewalls of nanowires to initiate new etching pathways to produce a porous structure. The elucidation of this fundamental formation mechanism opens a rational pathway to the production of wafer-scale single crystalline porous silicon nanowires with tunable surface areas ranging from 370 to 30 m(2) g(-1) and can enable exciting opportunities in catalysis, energy harvesting, conversion, storage, as well as biomedical imaging and therapy.     

Reduction of lattice thermal conductivity by point defects at intermediate temperatures

The lattice thermal conductivity is reduced by point defects because they scatter phonons. An analytic expression can be derived only in the limit of high temperatures; at lower temperatures one must have recourse to numerical calculations. Because the conductivity is due mainly to phonons of low frequencies when point-defect scattering is strong, the high-temperature approximation can be used at temperatures above half the Debye temperature. Numerical calculations, using the Ge-Si system as an example, show that the error incurred by using the high-temperature approximation is less than 10%.     

Enhancement of thermal conductivity of hydrogenated silicon film by microcrystalline structure growth

Hydrogenated silicon film is fabricated by plasma enhanced chemical vapor deposition method, and the enhancement of thermal conductivity of hydrogenated silicon film by microcrystalline structure growth is investigated. The thermal conductivity of films is measured based on Fourier thermal transmitting law by using platinum electrode. Raman spectroscopy characterization reveals the crystalline volume fraction (X _c) of microcrystalline silicon (μc-Si:H) and demonstrates it is embedded with nanocrystals. Spectroscopic ellipsometry with Forouhi–Bloomer model is used to obtain the thickness of films. The measurement results show that the thermal conductivity of μc-Si:H is much higher than amorphous silicon (a-Si:H).     

Subsurface damage of single crystalline silicon carbide in nanoindentation tests

The response of single crystalline silicon carbide (SiC) to a Berkovich nanoindenter was investigated by examining the indents using a transmission electron microscope and the selected area electron diffraction technique. It was found that the depth of indentation-induced subsurface damage was far larger than the indentation depth, and the damaging mechanism of SiC was distinctly different from that of single crystalline silicon. For silicon, a broad amorphous region is formed underneath the indenter after unloading; for SiC, however, no amorphous phase was detected. Instead, a polycrystalline structure with a grain size of ten nanometer level was identified directly under the indenter tip. Micro cracks, basal plane dislocations and possible cross slips were also found around the indent. These finding provide useful information for ultraprecision manufacturing of SiC wafers.     

Dislocation controlled wear in single crystal silicon carbide

For better design and durability of nanoscale devices, it is important to understand deformation in small volumes and in particular how deformation mechanisms can be related to frictional response of an interface in the regime where plasticity is fully developed. Here, we show that when the size of the cutting tool is decreased to the nanometer dimensions, silicon carbide wears in a ductile manner by means of dislocation plasticity. We present different categories of dislocation activity observed for single asperity sliding on SiC as a function of depth of cut and for different sliding directions. For low dislocation density, plastic contribution to frictional energy dissipation is shown to be due to glide of individual dislocations. For high dislocation densities, we present an analytical model to relate shear strength of the sliding interface to subsurface dislocation density. Furthermore, it is shown that a transition from plowing to cutting occurs as function of depth of cut and this transition can be well described by a macroscopic geometry-based model for wear transition.     

Transfer of single crystalline silicon nanolayer onto alien substrate

Starting from the 60-nm node, future generations of mainstream semiconductor devices (i.e., CMOS) will be mostly manufactured from silicon-on-insulator (SOI) initial substrates with the top silicon layer having a thickness 50 nm or less. We describe a process that is capable for transfer of nanoscale thick layers. The layer is delaminated from a single crystalline silicon substrate and laminated onto another substrate, thus resulting in SOI. The process includes: 1) forming a trap layer for hydrogen in an initial substrate; 2) delivery of hydrogen to the traps by diffusion of monatomic hydrogen; 3) evolving the trapped hydrogen into a layer of hydrogen platelets; 4) stiffening of the surface of the initial substrate by laminating to another substrate; and 5) delaminating a layer from the initial substrate along the hydrogen platelet layer. Details of the new layer transfer process are described. A depth where the buried trap layer locates is critical for the process. An implantation of heavy ions is used to form the trap layer. A trap capacity for hydrogen is evaluated as a function of implantation conditions. Plasma hydrogenation is used to deliver atomic hydrogen to the traps. Electron cyclotron resonance, microwave, RF, and dc plasma are compared as the hydrogenation sources. Dependence of a thickness of a transferred layer as a function of the mass of implanted ions and implantation energy is described. Types of layer transfer faults are also described. Mechanisms of the layer transfer faults are suggested. We discuss limits of scaling down of the thickness of the layer that is transferred from one substrate to another. The scaling limit of our process is compared to the limits of other (SIMOX, Smart-Cut, and ELTRAN) processes.     

单晶硅表面溶胶-凝胶法制备磷灰石涂层的研究

采用溶胶-凝胶法在单晶硅表面制备了磷灰石(apatite,AP)涂层。借助SEM、EDS、XRD研究了单晶硅表面AP涂层的形貌结构与成分特征,通过模拟体液(simulated body fluid,SBF)浸泡实验研究了硅基AP涂层的体外诱导性能。研究结果表明,在500℃保温2h即可使sol-gel层转变为较致密的AP涂层,700与800℃分别保温2h得到的硅基AP涂层晶体长大且出现间隙,涂层的结晶性随热处理温度升高而增加,在800℃处理未出现AP的分解;SBF中浸泡1周后,AP涂层未出现剥离单晶硅的现象,且能够诱导骨状磷灰石新沉积层的形成,体现出较好的体外诱导性能。     

Dissociating the phononic,magnetic and electronic contributions to thermal conductivity: a computational study in alpha-iron

Journal of Materials Science - Computational tools to study thermodynamic properties of magnetic materials have, until recently, been limited to phenomenological modeling or to small domain sizes...     

Relationship between thermal diffusion and conductivity for two-component crystalline structures

This paper gives a more accurate form of Reinhold's formula, which connects the concentration thermal diffusion effect with the transport number and electrical conductivity. The obtained formula correctly predicts the direction of thermal diffusion and the order of magnitude, but the results are a little too high.     

Electrical conductivity of single crystal and polycrystalline yttria-stabilized zirconia

The conductivity of several single crystal and polycrystalline Y₂O₃-ZrO₂ samples has been studied by complex impedance and four-probe direct current techniques. For single crystals only one arc, due to lattice conductivity, was observed in the complex impedance representation. Polycrystalline materials showed a second arc, due to grain boundary resistance, the extent of which decreased as the impurity concentration was reduced and as the electrolyte microstructure improved. The activation energies for the volume and total conductivity of the purest polycrystalline samples were similar and agreed with those for the single crystals. These values, however, decreased by 20 to 25 kJ mol^–1 on going from low (<550° C) to high (>850° C) temperatures. The change in the activation energy with temperature is thought to be due to a gradual transition between an association region, where vacancies are bound to dopant cations, and a dissociation region where vacancies are free and mobile.     

Contribution of crystalline lattice defects to thermal conductivity of porous materials

The relationship of pore thermal resistance and effective thermal conductivity of porous media to the processes of crystalline lattice defect formation and motion is demonstrated.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 44, No. 6, pp. 965–969, June, 1983.     

Energy considerations in crack deflection phenomenon in single crystal silicon

Crack deflection in single-crystal brittle occurs when a crack, propagating on one cleavage plane, ‘chooses’, from energy considerations, to continue propagating on another cleavage plane. This phenomenon was identified during dynamic crack propagation experiments of thin, rectangular [0 0 1] single-crystal (SC) silicon specimens subjected to three-point bending (3PB). Specimens with long pre-cracks (hence propagating at a ‘low’ energy and velocity) cleave along the vertical (1 1 0) plane, while the same specimens but with short pre-cracks (and therefore with higher propagation energy and velocity) cleave along the inclined (1 1 1) plane. The same specimens with intermediate pre-crack length show that the crack first propagates on the (1 1 0) plane and then deflects to the (1 1 1) plane. We show that the deflection is due to variations of the material property that resists cracking, Γ, the dynamic cleavage energy, with velocity and crystallographic orientation. We propose selection criteria to explain the deflection: The crack will deflect to the plane with the lowest dynamic cleavage energy. We further suggest that crack deflection is the basic mechanism controlling the way the crack consumes energy while propagating and is the main cause of surface perturbations. The spatial temporal fracture energy along the (1 1 0) cleavage plane is evaluated.     

Optical characterization of porous silicon and crystalline silicon by the Kramers-Kronig method

The electronic structure of porous silicon (PS) has been characterized by optical reflectance spectra analyses. Using a Cary-500 spectrometer, the reflectance spectra of PS are measured in the photon energy range of 0.4-6 eV. The spectral responses of optical constants are calculated for PS and Si by Kramers-Kronig analysis. The analysis clarified strong evidence for widening and direct bandgaps for PS samples. Also, the optical constants of PS layers as a function of porosity have been studied. Our results indicate that PS retains some of the characteristic optical features of crystalline Si. However, in the visible region, PS shows that the imaginary part of the complex refractive index increases, and the real part decreases as porosity increases. This feature could be related to the surface roughness of PS and its role in surface absorption and scattering.