Single Crystalline Silicon Wafers Research Articles

Nanomechanical Properties of Single-Crystalline Silicon Wafer Irradiated by High-Energy Femtosecond Laser Pulses

High-energy femtosecond laser pulses were utilized to ablate single-crystalline silicon wafer. Collateral damage areas around the ablation zone can be observed in microscope. The morphology in such areas changes gradually. The microscopic morphology and nanomechanical properties of the pre-polished back surface were measured by AFM and Hysitron TriboIndenter respectively. The topography and roughness in the ablated, metamorphic and unaffected zone are almost equal. Yet the elastic ratio and hardness on the back surface vary gradually with indent positions, which coincide with the gradual morphological changes in the metamorphic zone on the front surface. Such regular changes in nanomechanical properties, to some extent, reflect the distribution of collateral damages near the ablated zone on the back surface. And they also testify the occurrence of the ill effects that go against micromachining during high-energy femtosecond laser irradiation.

Selective growth of carbon nantoubes on SiO 2/Si substrate

Three-step raising temperature process was employed to fabricate carbon nanotubes by pyrolysis of ferrocene/melamine mixtures on silica and single crystalline silicon wafers respectively. Then the morphologies, structures and compositions of obtained carbon nanotubes are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscope (EDX) and electron energy-loss spectroscopy (EELS). TEM and SEM observation shows that on silica substrate, high-oriented carbon nanotube can grow compactly to form continuous film on both frontal and cross-section surfaces, but on silicon substrate, only can form on cross-section surface. These carbon nanotubes have much irregular cup-like structure, and with outer diameter varying from 25 nm to 35 nm. At the top end of carbon nanotube there is a catalyst particle. EDX analysis reveals that the particle are iron cluster, and EELS spectrum indicates that the nanotube is composed of pure carbon. Finally, the effect of substrate surface roughness on the growth behavior of carbon nanotubes has been discussed.

Identification of material and geometrical parameters for microstructures by dynamic finite element model updating

This paper describes a procedure to identify material and geometrical parameters for microstructures using the concept of finite element model updating. This scheme utilizes measured and finite element analysis (FEA) natural frequencies that are paired together according to their mode shapes, and it incorporates an optimization sequence that formulates the frequency differences as an error vector to be minimized. To demonstrate the effectiveness of the proposed procedure, two examples are shown in this paper. One example involves a microcantilever fabricated from a single-crystalline silicon wafer, and the updating process is applied on the cantilever to identify its Young’s modulus. The identified Young’s modulus (along direction) of 130.29 GPa is very comparable to those in the literature. The other example concerns a commercial, V-shaped silicon nitride probe used in an atomic force microscope. The natural frequencies and mode shapes of the probe are measured, the FEA performed, and the probe thickness and the Young’s modulus of the silicon nitride substrate determined. The identified thickness is also verified by SEM images of the probe. Both examples show that the updating procedure converged in just a few iterations.

Reproduction of mouse-pup ultrasonic vocalizations by nanocrystalline silicon thermoacoustic emitter

As one of the functional properties of ultrasound generator based on efficient thermal transfer at the nanocrystalline silicon (nc-Si) layer surface, its potential as an ultrasonic simulator of vocalization signals is demonstrated by using the acoustic data of mouse-pup calls. The device composed of a surface-heating thin-film electrode, an nc-Si layer, and a single-crystalline silicon (c-Si) wafer, exhibits an almost completely flat frequency response over a wide range without any mechanical surface vibration systems. It is shown that the fabricated emitter can reproduce digitally recorded ultrasonic mouse-pups vocalizations very accurately in terms of the call duration, frequency dispersion, and sound pressure level. The thermoacoustic nc-Si device provides a powerful physical means for the understanding of ultrasonic communication mechanisms in various living animals.

Mechanism of a remarkable enhancement in the light emission from nanocrystalline porous silicon annealed in high-pressure water vapor

To clarify the effect of surface passivation on the optical properties of nanocrystalline porous silicon (PS), the photoluminescence (PL) characteristics of PS have been investigated by employing a high-pressure water vapor annealing (HWA). PS samples with various porosities were prepared on (100)-oriented p-type (4Ωcm) single-crystalline silicon wafers by electrochemical anodization. Some samples were then electrochemically oxidized. The HWA treatment was then applied to the prepared PS samples at 0.5–3MPa and 200–300°C for 2–3h. The PL intensities, spectra, and dynamics after HWA were measured in relation to surface analyses by Fourier-transform-infrared (FTIR) spectroscopy. It is shown that the HWA treatment leads to a drastic enhancement in both the PL efficiency and stability. Under the optimum condition, the PS sample exhibits an extremely high external quantum efficiency of 23% at room temperature. According to the FTIR spectra analyses, silicon nanocrystallites in HWA-treated PS are covered with a high-quality SiO2 tissue. The PL decays are found to be longer than those of as-prepared PS, and become closer to a single-exponential behavior near the PL peak wavelength. The observed high efficiency and stability of PL emission from HWA-treated PS is attributed to (i) suppression of nonradiative surface defect density, (ii) uniform passivation by unstrained thin oxides, and (iii) strong localization of excitons in silicon nanocrystals. This low-temperature treatment is very useful for obtaining highly efficient and stable luminescent PS and devices.

Electrical properties of B-doped polycrystalline silicon thin films prepared by rapid thermal chemical vapour deposition

In this paper, about 30 μm thick B-doped polycrystalline silicon (poly-Si) thin films were deposited on quartz substrates, n-type single crystalline silicon wafers and p++-type poly-Si ribbons by a rapid thermal chemical vapour deposition system in a temperature range from 1000 to 1150 °C. Activation energy measurement and room temperature/temperature dependent Hall effect measurement were performed on the poly-Si thin films prepared on the former two kinds of substrates, respectively. It seems that the electrical properties of as-prepared poly-Si thin films could be qualitatively explained by Seto's grain boundary (GB) trapping theory although there is a big difference between our samples and Seto's in grain size and film thickness etc. The experimental results reconfirm that GB itself is a kind of most effective recombination center with trapping level near the midgap and trapping state density in the order of 1012 cm−2 magnitude. Electron beam induced current measurements on the poly-Si thin films prepared on the poly-Si ribbons also show that severe recombination occurs at the positions of GBs.

Vertical micromirrors integrated with electromagnetic microactuators for two-dimensional optical matrix switches

We report novel single crystalline silicon (SCS) micromirrors vertically attached to electromagnetic microactuators for the two-dimensional (2-D) free-space optical cross-connect switching application. The operation voltage is typically 0.7 V due to the large electromagnetic force generated by the coils on the microactuators. Vertical micromirrors with the surface area of 500×1200 μm are monolithically integrated with microactuators. The micromirrors are fabricated by low-cost tetra-methyl ammonium hydroxide anisotropic wet etching on the [110] SCS wafers. The surface roughness of the micromirror is less than 20 nm and the measured insertion loss caused by the micromirror is approximately 0.2 dB. The large surface area of the micromirror can accommodate a big optical beam for 2-D matrix configuration to minimize insertion loss. Chips consisting of 10 × 10 arrays are successfully fabricated and tested.

Nanocrystalline ZnO thin films on porous silicon/silicon substrates obtained by sol–gel technique

In this study, nanocrystalline ZnO thin films deposited onto silicon substrates by sol–gel technique were studied. The starting material for ZnO sol–gel thin films was Zinc acetate. Single crystalline silicon wafers with different orientations and the porous silicon substrates were used as the substrates. XRD was employed to investigate the evolution of the crystalline orientation during the thermal treatment, and SEM was used to observe the surface morphology of the ZnO films. The effects of the anneal temperature and the substrate doping type on the deposited ZnO thin films were studied in detail. The results indicated that the highly oriented ZnO could be generated on Si substrates by controlling the process conditions. It was found that the substrates types had less influence on the obtained ZnO thin films compared with the anneal temperatures. Thin porous silicon substrates introduced is beneficial to enhance the bonding strength between the films and the substrates. These results are helpful to deposit the highly orientation-controlled ZnO thin films for different kinds of research and application.

Optical properties of (Al2O3)x(TiO2)1−x films deposited by the sol–gel method

The optical constants (refractive index, n and absorption coefficient, k) in the visible and near-UV spectral range of (Al2O3)x(TiO2)1−x films, deposited by the sol–gel method on polished single-crystalline silicon wafers, was studied at varied ratios of the components Al2O3 :TiO2. The thickness was determined by ellipsometry at 632.8nm and a multiwavelength curve fitting method was applied to calculate the refractive index dispersion. The absorption coefficient α and optical band gap Eg were determined from the transmission and reflection spectra when a sapphire substrate was used.

The effect of baking conditions on the effective contact areas of screen-printed silver layer on silicon substrate

In this paper, Ag-based paste was screen-printed on the polished as well as on the textured p-type (100) single crystalline silicon wafers. Three types of baking processes were studied: the tube furnace, the belt furnace and the hot plate baking. The effective contact areas of Ag/Si system were measured with a novel method, namely metal insulator semiconductor structure measurement. The results show that after baking on the hot plate at 400°C for 5 min, the size and number of pores in the Ag film layer as well as at the interface between silver layer and silicon decreases significantly, the effective contact area also increases about 20%, particularly on the textured silicon substrate.

Quantitative analysis of osteoblast behavior on microgrooved hydroxyapatite and titanium substrata.

The effects of implant surface topography and chemistry on osteoblast behavior have been a research focus because of their potential importance in orthopedic and dental applications. This work focused on the topographic effects of hydroxyapatite (HA) and titanium (Ti) surface that had identical micropatterns to determine whether there was synergistic interaction between surface chemistry and surface topography. Surface microgrooves with six different groove widths (4, 8, 16, 24, 30, and 38 microm) and three different groove depths (2, 4, and 10 microm) were made on single crystalline silicon wafers using microfabrication techniques. Ti and HA thin films were coated on the microgrooves by radio-frequency magnetron sputtering. After that, human osteoblast-like cells were seeded and cultured on the microgrooved surfaces for up to 7 days. The cells' behavior was examined using scanning electron microscopy after cells were fixed and dehydrated. Statistical analysis was based on quantitative data of orientation angle, evaluating the contact guidance, and form index, describing cell shape or cell morphology changes. The contact guidance and cell shape changes were observed on the HA and Ti microgrooves. No difference in orientation angle between HA and Ti microgrooves was found. This might suggest that surface chemistry was not a significant influence on cell guidance. However, the form index analysis indicated an interaction between topographic effects and surface chemistry. Thus, conclusions about surface topographic effects on cell behavior drawn from one type of material cannot simply be applied to another type of material.

Rectifying and photovoltaic effects observed in mesoporous MCM-41 silica film on silicon

Oriented mesoporous MCM-41 silica films have been synthesized on single-crystalline silicon (111) wafers by the self-assembly deposition in ammonia medium. For the first time, the I– V curves and surface photovoltaic spectra of the films has been investigated. The I– V curve of the as-synthesized film exhibits an excellent rectified ratio of σ +/ σ −=3×10 3 (where σ +, σ − are the forward electrical conductivity and reverse electrical conductivity, respectively). The results of photovoltaic measurements indicate that the as-synthesized film possesses a significant surface photovoltaic increase.

Phase Transformation of Single Crystalline Silicon by Scratching

The relationship between the phase transformation and the stress applied on single-crystalline silicon wafer was studied by performing a scratch test. The cracks and phases were observed by optical microscopy and micro Raman spectroscopy, respectively. Various silicon polymorphs and crack shapes were observed as functions of speed and direction of the scratching, respectively. Atomic force microscopy was also carried out to analyze the morphology of the scratch grooves.

2-D Mesoparticulate Arrays of α-Cr2O3

An inexpensive low-temperature aqueous thin film processing technique is presented for the fabrication of large two-dimensional (2-D) arrays of α-Cr2O3 without template, surfactant, or applied external field. The arrays consist of well-defined, monodispersed, and noninteracting crystalline spherical mesoparticles (250 nm) that are grown directly onto single crystalline silicon wafers (Si or SiO2) or polycrystalline transparent conducting oxide (TCO) substrates by heteronucleation. Their UV−visible optical absorption properties are consistent with the electronic transitions of Cr3+ ions in D3d symmetry but significantly blue-shifted compared to single crystal or polydispersed α-Cr2O3 samples. Such arrays could be of interest for theoretical and experimental studies of the unique nonreciprocal optical effects observed in α-Cr2O3.

Deposition of diamond/β–SiC Gradient Composite Films by Microwave Plasma-assisted Chemical Vapor Deposition

Mixed-phase diamond/β–SiC composite films with compositional gradient were prepared by microwave plasma-assisted chemical vapor deposition using a gas mixture of hydrogen, methane and tetramethylsilane (TMS). Single-crystalline silicon wafers, pretreated with diamond nanoparticles before deposition, were used as substrates. The film characterization by scanning electron microscopy, electron probe microanalysis, and energy-dispersive x-ray analysis shows that the contents of diamond and silicon carbide in the films vary with TMS flow rate. Diamond/β–SiC composite films with compositional gradients are achievable by varying the TMS flow rate during the film growth process.

Gettering of impurities in solar silicon

Electrical properties of crystalline silicon wafers used for photovoltaı̈cs are degraded by metallic impurity atoms. Such atoms are introduced during the crystal growth or during the processing steps needed to prepare solar cells. External gettering treatments such as phosphorus diffusion from a POCl 3 source or Al–Si alloying are needed to restore or to improve the bulk electrical properties of the material. Monocrystalline wafers can be easily ugraded by such treatments. In multicrystalline silicon wafers, external gettering by phosphorus diffusion, as well as by Al–Si alloying are efficient, provided the temperature does not exceed 900°C. Longer treatments (2–4 h) are needed in order to increase the minority carrier diffusion length beyond the wafer thickness. So longer times are necessary to dissolve metallic atom containing precipitates. However, if the major part of the wafer is neatly improved, some regions containing dislocation tangles are poorly modified. In such regions, impurities could be involved in the formation of silicates which cannot be dissolved during the gettering treatment. Nevertheless, external gettering treatments are able to clean efficiently single crystalline and multicrystalline silicon wafers, provided the oxygen concentration and the defect density are not too high and are homogeneously distributed.

Surface passivation of p-type crystalline Si by plasma enhanced chemical vapor deposited amorphous SiCx:H films

Excellent passivation properties of intrinsic amorphous silicon carbide (a-SiCx:H) films deposited by plasma enhanced chemical vapor deposition on single-crystalline silicon (c-Si) wafers have been obtained. The dependence of the effective surface recombination velocity, Seff, on deposition temperature, total pressure and methane (CH4) to silane (SiH4) ratio has been studied for these films using lifetime measurements made with the quasi-steady-state photoconductance technique. The dependence of the effective lifetime, τeff, on the excess carrier density, Δn, has been measured and also simulated through a physical model based on Shockley–Read–Hall statistics and an insulator/semiconductor structure with fixed charges and band bending. A Seff at the a-SiCx:H/c-Si interface lower than 30 cm s−1 was achieved with optimized deposition conditions. This passivation quality was found to be three times better than that of noncarbonated amorphous silicon (a-Si:H) films deposited under equivalent conditions.

Growth of Oxygen Precipitates in Low-Dose Low-Energy Simox

Silicon-on-insulator (SOI) is becoming a key technology for low power electronics due to substantially reduced power consumption of electronic components, and a capability of compact circuit design which are not readily achievable in bulk silicon technology [1]. Separation by IMplantation of OXygen (SIMOX) is the most promising technology for fabricating SOI material. The basic SIMOX process consists of implantation of oxygen into the single crystalline silicon wafer and the subsequent high temperature annealing. Oxygen implantation at low doses does not form a continuous buried oxide (BOX) layer but leads to an inhomogeneous distribution of the oxygen precipitates during implantation process. The formation and growth of oxygen precipitates in low-dose SIMOX depend strongly on the implantation conditions such as oxygen dose, implantation temperature, annealing temperature and ramping rate [2,3]. During the subsequent annealing, Ostwald ripening of the precipitates takes place and the larger precipitates grow at the expanse of small ones until they coalesce to the buried oxide layer.

Engineering analysis of microdefect formation during silicon crystal growth

The quality of single crystalline silicon wafers used as substrates for microelectronic devices is measured in terms of the type, size and number density of microdefects formed during crystal growth and subsequent processing. Native point defects—vacancies and self-interstitials—and impurities, such as oxygen and carbon, in the silicon crystal convect, diffuse, react and aggregate, driven by species super-saturation, to form defect structures at densities that are microscopically visible. We model defect dynamics using diffusion–reaction theory in which defect types are represented as chemical species and have concentrations that are governed by conservation laws, chemical thermodynamics and kinetics, using self-consistent values of equilibrium, transport and kinetic parameters for point defects and impurities. We refer to this approach as crystal defect dynamics analysis. The purpose of this paper is to demonstrate that this approach leads to a wide range of predictions that are consistent with experiments. The results presented here focus on the two-dimensional predictions of microdefect structures under growth conditions where both voids (cluster of vacancies) and self-interstitial aggregates are seen in regions of the crystal separated by the oxidation-induced stacking fault ring (OSF-ring). The location and structure of the OSF-ring is predicted in terms of point defect dynamics near the melt/crystal interface, which results in an annular region with almost balanced point defect concentrations (centered at r≡RΔ=0), and in terms of a peak (centered at r≡RCV,max) in the residual vacancy concentration after cluster formation. This residual vacancy concentration facilitates oxide precipitation during crystal growth, which seeds stacking fault formation during subsequent oxidation. Calculations show that RCV,max⩽RΔ=0 with both values scaling with V/G(R), where V is the crystal pull rate and G(R) is the axial temperature gradient at r=R along the melt/crystal interface. An annulus just outside the OSF-ring (centered at r≡Rfree,RCV,max⩽Rfree⩽RΔ=0) is identified where almost no microdefects are present, thereby identifying the temperature field that leads to almost perfect silicon.

Cathodoluminescence from Nanocrystals in Mechanically Milled Silicon

Mechanical milling is a technique extensively used to prepare nanocrystalline metallic materials but it has been less frequently applied to semiconductors. The preparation of nanocrystalline silicon by different methods is a subject of increasing interest due to the luminescent properties of this material and its possible application in optoelectronic devices. In this work high-energy ball milling has been used to prepare nanocrystals from single crystalline silicon wafers. The structure of the milled samples has been assessed by X-ray diffraction, transmission electron microscopy (TEM) and Raman spectroscopy, while their luminescence has been investigated by cathodoluminescence (CL) in the scanning electron microscope (SEM). The samples consist of a powder of particles with sizes of hundreds of nanometers aggregated to form bigger particles of several microns. TEM reveals that the particles consist of nanocrystals with a wide range of sizes including crystallites with dimension of few nanometers. CL spectra of the milled samples show a band at 1.61 eV, attributed to the presence of nanocrystals through a quantum confinement effect. It is suggested that a milling induced infrared emission is related to a high density of extended defects present in the milled samples.