Structure of natural film on porous silicon surface founded by ellipsometry

Abstract

The multiple-angle-of-incidence ellipsometric studies of the natural film formed on the surface of porous silicon sample after its production and subsequent keeping in isopropyl alcohol have been performed. A new property of porous silicon with the natural film on its surface has been revealed which consists in a drastic change of the polarization characteristics of the reflected light wave after its keeping in alcohol. This property of porous silicon may be the basis for the development of liquid and gas sensors, polarization elements of optical devices etc. It was found that ellipsometric curves are described better by calculations in a two-layer model in comparison with a single-layer one of the investigated film. It was obtained that the outer layer of the film has smaller refractive index than the inner one. As a result of decoration the refractive indices of both layers decrease and further the changes of refractive indices and the redistribution of the thickness of both layers are found. It is suggested that the interaction of the alcohol molecules with silicon and its oxide environment occurs in the walls of the pores during the decoration.     

Plasma‐enhanced chemical vapor deposition of 99.95% 28Si in form of nano‐ and polycrystals using silicon tetrafluoride precursor

The process of plasma chemical deposition of silicon was investigated from its tetrafluoride containing 99.99% of ²⁸Si isotope in the form of thin layer of nano‐crystalline silicon on silicon substrate and of thick layer of polycrystalline silicon on the inner surface of quartz reactor. The layers are characterized by the methods of X‐ray diffraction and Raman spectroscopy. Using the SIMS method the mechanism of isotopic dilution was investigated in the PECVD process (the content of ²⁸Si isotope in layers was 99.95‐99.98%). A necessity is indicated in thorough special preparation of the reactor for minimization of isotopic dilution in case of fabrication of silicon containing ≥99.9% of ²⁸Si. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photosensitive organic-inorganic hybrid structures based on porous silicon

Abstract

In this study, the photovoltaic organic-inorganic structures were created by deposition of poly(3,4-ethylenedioxythiophene) film doped by poly(styrenesulfonate) and reduced graphene oxide on the porous silicon/silicon substrate. Formation of the hybrid structure was confirmed by means of atomic-force microscopy and Fourier transform infrared spectroscopy. The current-voltage characteristics of the obtained structures were studied. It was found the increase of electrical conductivity and photo-induced signal in organic-inorganic structures. Temporal parameters and spectral characteristics of photoresponse in the 400–1100?nm wavelength range were investigated. The widening of spectral photosensitivity in a short-wavelength range due to light absorption in various layers of the multijunction structure in comparison with single crystal silicon was revealed.     

Structural and optical properties of vapour‐etching based porous silicon

The structural and optical properties of porous silicon (PS) layers prepared by Vapour‐Etching (VE) of moderately and heavily boron‐doped Si substrates are investigated. The VE technique produces rough PS layers that are essentially formed of interconnected cluster‐like structures. Optical investigations indicate that this surface roughness enables the PS layers to be used as antireflection coatings in silicon based devices. These optical characteristics are investigated by optical reflectivity and light scattering. The local chemical state and the microstructure of the PS layers are studied by electron energy loss spectroscopy (EELS) in transmission electron microscopy (TEM), and are correlated to the red photoluminescence (PL). TEM studies point out that the cluster‐like interconnected structures are composed of luminescent nanocrystallites. PL measurements display that both quantum confinement and surface passivation determine the electronic states of the silicon nanocrystallites. The complex dielectric function is calculated from the experimental single‐scattering distribution spectrum using a Kramers Kronig analysis. The first resonance peak in the imaginary part is observed at 2.3 eV; two other broadened features appear at 4.7 and 8.8 eV. The latter is generally related to an interface plasmon resulting from the silicon‐silicon oxide interface. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Complex boron redistribution kinetics in strongly doped polycrystalline‐silicon/nitrogen‐doped‐silicon thin bi‐layers

We have investigated the complex behaviour of boron (B) redistribution process via silicon thin bi‐layers interface. It concerns the instantaneous kinetics of B transfer, trapping, clustering and segregation during the thermal B activation annealing. The used silicon bi‐layers have been obtained by low pressure chemical vapor deposition (LPCVD) method at 480 °C, by using in‐situ nitrogen‐doped‐silicon (NiDoS) layer and strongly B doped polycrystalline‐silicon (P⁺) layer. To avoid long‐range B redistributions, thermal annealing was carried out at relatively low‐temperatures (600 °C and 700 °C) for various times ranging between 30 min and 2 h. To investigate the experimental secondary ion mass spectroscopy (SIMS) doping profiles, a redistribution model well adapted to the particular structure of two thin layers and to the effects of strong‐concentrations has been established. The good adjustment of the simulated profiles with the experimental SIMS profiles allowed a fundamental understanding about the instantaneous physical phenomena giving and disturbing the complex B redistribution profiles‐shoulders. The increasing kinetics of the B peak concentration near the bi‐layers interface is well reproduced by the established model.     

Dislocation‐related photoluminescence imaging of mc‐Si wafers at room temperature

We report on a method for fast detection of defect rich areas in multicrystalline silicon solar wafers. It is based on photoluminescence imaging of the whole wafers and detects both the band‐to‐band radiation as well as the dislocation specific radiation D1. To illustrate the capabilities of the method we examined 5.0 × 5.0 cm² wafer pieces in different stages of their processing. The achieved resolution of the D1 images was ∼120 μm, within a total recording time of 550 ms.     

In-situ Raman spectroscopy used to study and control the initial growth phase of microcrystalline absorber layers for thin-film silicon solar cells

The continuous deposition of microcrystalline silicon has been monitored with in-situ Raman spectroscopy. The process and measurement settings were chosen such that one spectrum was taken during approximately 9 nm of layer growth. This allows observing the crystallinity in the initial growth phase of microcrystalline silicon absorber layers. The influence of different p-doped seed layers has been studied. Under constant deposition conditions, an initial decrease in crystallinity was observed over the first tens of nanometers. By profiling the process gas flows during the initial phase it was possible to reduce the amount of amorphous material that was detected during the initial phase of deposition.     

Oxygen and carbon transfer during solidification of semiconductor grade silicon in different processes

A model is established for comparing the solute distribution resulting from four solidification processes currently applied to semiconductor grade silicon: Czochralski pulling (CZ), floating zone (FZ), 1D solidification and electromagnetic continuous pulling (EMCP). This model takes into account solid–liquid interface exchange, evaporation to or contamination by the gas phase, container dissolution, during steady-state solidification, and in the preliminary preparation of the melt. For simplicity, the transfers are treated in the crude approximation of perfectly mixed liquid and boundary layers. As a consequence, only the axial (z) distribution can be represented. Published data on oxygen and carbon transfer give a set of acceptable values for the thickness of the boundary layers. In the FZ and EMCP processes, oxygen evaporation can change the asymptotic behaviour of the reference Pfann law. In CZ and in 1D-solidification, a large variety of solute profile curves can be obtained, because they are very sensitive to the balance between crucible dissolution and evaporation. The CZ process clearly brings supplementary degrees of freedom via the geometry of the crucible, important for the dissolution phenomena, and via the rotation rate of the crystal and of the crucible, important for acting on transfer kinetics.     

Electrical and sensory properties of zinc oxide – porous silicon nanosystems

Abstract

Zinc oxide nanostructures have been grown by electrochemical deposition on porous silicon–silicon substrate. The photoelectric and sensory properties of the obtained ZnO–porous silicon nanosystems were investigated in both DC and AC regimes. The obtained structures were characterized by photosensitivity in the 400–1100?nm wavelength range and by high sensitivity to moisture. Increase of relative humidity resulted in significant decrease of the electrical resistance and increase of the capacitance of the hybrid structures. To estimate the sensory properties of the ZnO–porous silicon nanostructures their adsorption sensitivity and dynamic characteristics were analyzed. Discovered features of the charge transport processes broaden the prospects of the semiconductor nanosystems application in gas sensors and photodetectors.     

Vapour phase growth of epitaxial silicon carbide layers

After a brief overview of different epitaxial layer growth techniques, the homoepitaxial chemical vapour deposition (CVD) of SiC with a focus on hot-wall CVD is reviewed. Step-controlled epitaxy and site competition epitaxy have been utilized to grow polytype stable layers more than 50 μm in thickness and of high purity and crystalline perfection for power devices. The influence of growth parameters including gas flow, C/Si ratio, growth temperature and pressure on growth rate and layer uniformity in thickness and doping are discussed. Background doping levels as low as 10¹⁴ cm⁻³ have been achieved as well as layers doped over a wide n-type (nitrogen) and p-type (aluminium) range.

Furthermore the status of numerical process simulation is mentioned and SiC substrate preparation is described. In order to get flat and damage free epi-ready surfaces, they are prepared by different methods and characterised by atomic force microscopy and by scanning electron microscope using channelling patterns. For the investigation of defects in SiC high purity CVD layers are grown. The improvement of the quality of bulk crystal substrates by micropipe healing and so-called dislocation stop layers can further decrease the defect density and thus increase the yield and performance of devices. Due to its high growth rate functionality and scope for the use of multi-wafer equipment hot-wall CVD has become a well-established method in SiC-technology and has therefore great industrial potential.     

溅射气压对碳硅氧薄膜透过率及光学带隙的影响

采用射频磁控溅射技术在玻璃基底和单晶硅片(100)上制备了碳硅氧(SiOC)薄膜,通过扫描电镜、X射线衍射、拉曼光谱、X射线光电子能谱及紫外可见透射光谱等技术手段对其进行了分析,研究了在不同溅射气压下所制备薄膜的组分、透过率及光学带隙.结果表明:随着溅射气压的增大,薄膜内部sp3键含量、透过率及光学带隙均随之增大,sp3键及其形成的宽带隙σ键对薄膜光学带隙有着较大影响.在溅射气压为3.0 Pa的条件下,薄膜光学带隙为2.67 eV.     

Influence of the deposition parameters on the microstructure and opto-electrical properties of hydrogenated nanocrystalline silicon films by HW-CVD

Hydrogenated nanocrystalline silicon (nc-Si:H) films were prepared at high deposition rates (> 13 Å/s) from pure silane without hydrogen dilution by hot wire deposition method by varying filament-to-substrate distance (d_s-f). In this study we have systematically and carefully investigated the effect of filament-to-substrate distance on structural, optical and electrical properties of the Si:H films. A variety of characterization techniques, including Raman spectroscopy, low angle X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Atomic Force Microscopy (AFM), Field Emission Scanning Electron Microscopy (FE-SEM), UV-Visible-NIR spectroscopy and electrical dark and photoconductivity measurement were used to characterize these films. Films deposited at d_s-f > 5 cm are amorphous while those deposited at d_s-f < 5 cm are biphasic; a crystalline phase and an amorphous phase with nano-sized crystallites embedded in it. Low angle X-ray diffraction analysis showed that the crystallites in the films have preferential orientation along (111) directions. Decrease in d_s-f, the crystallinity and crystalline size increases whereas hydrogen bonding shifts from mono-hydride (SiH) to di-hydride (SiH₂) and poly-hydride (SiH₂)_n complexes. The band gaps of nc-Si:H films (~ 1.9-2.0 eV) are high compared to the a-Si:H films, while hydrogen content remains < 10 at.%. We attribute the high band gap to the quantum size effect. A correlation between electrical and structural properties has been established. Finally, from the present study it has been concluded that the filament-to-substrate distance is a key process parameter to induce the crystallinity in the films by hot wire method. The ease of depositing films with variable crystallite size and its volume fraction, and tunable band gap is useful for fabrication of tandem/micro-morph solar cells.     

Comparison of growth methods for Si/SiO2 nanostructures as nanodot hetero-emitters for photovoltaic applications

Two different growth mechanisms are compared for the fabrication of Si/SiO₂ nanostructures on crystalline silicon (c-Si) to be used as hetero-emitter in high-efficiency solar cells: (1) The decomposition of substoichiometric amorphous SiO_x (a-SiO_x) films with 0 < x < 1.3 and (2) the dewetting of thin amorphous silicon (a-Si) layers.The grown layers are investigated with regard to their structural properties, their passivation quality for c-Si wafer substrates and their electrical properties in order to evaluate their suitability as a nanodot hetero-emitter. While by layer decomposition, no passivating nanodots could be formed, the dewetting process allows fabricating nanodot passivation layers at temperatures as low as 600 °C. The series resistance through Ag/[Si-nanodots in SiO₂]/c-Si/Al structures for dewetting is similar to nanostructured silicon rich SiO_x films. Still, a nanodot hetero-emitter which exhibits both a satisfying passivation of the substrate and induces a high band bending by doping at the same time could not be fabricated yet.     

PECVD growth of crystalline silicon from its tetrafluoride

The process of plasma chemical deposition of silicon from inductively coupled plasma of the mixture of high‐purity SiF₄ and H₂ sustained by RF discharge at 13.56 MHz in amount sufficient for subsequent growth of crystal by Czochralski method was investigated. The structure and impurity content of the produced layers as well as of the grown crystal have been studied (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

高压13.56MHz PECVD法沉积器件质量级本征微晶硅材料

本文国内首次报道了采用高压RF-PECVD技术沉积本征微晶硅材料的结果.实验表明,增大等离子体激发功率和减小硅烷浓度都能够使薄膜材料由非晶硅逐渐向微晶硅转变,而结构上的改变使得电学特性也随之改变.通过工艺参数的优化和纯化器的使用,有效地控制了氧的掺杂,在较高的生长速度下得到了器件质量级的本征微晶硅材料.将实验得到的微晶硅作为太阳电池光吸收层,在没有ZnO背电极和没有优化窗口层材料以及p/i界面时,电池的效率达到5.22;,这进一步表明本征微晶硅材料的良好性能.     

Crystallization of silicon thin films by current‐induced joule heating using a tungsten overcoat

A modified crystallization process using current‐induced joule heating under vacuum is presented. A thin layer of high temperature resistant tungsten was sputtered on the amorphous silicon as the conducting and annealing medium. The thin film thickness was measured by α‐stepper. The high current density provided effective means in crystallizing the amorphous silicon layer. The crystalline morphology was studied by scanning electron microscopy (SEM) after Secco‐etch, transmission electron microscopy (TEM), and x‐ray diffraction (XRD), under different annealing conditions. The grain size was controlled in the range of 0.1‐0.5 μm and could be increased with annealing time. No tungsten silicide was found. Some defects were formed due to electron‐migration effect near the electrodes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

脉冲激光沉积碳薄膜生长中氢气的作用

采用石墨靶,通过脉冲激光沉积技术在温度为20℃的玻璃和硅衬底上沉积出厚度为100nm的碳薄膜,所用的激光源是ArF激光(λ＝193nm, 24ns). 通过调节氢气流量使反应室的压力在1.33×10-5～133Pa之间变化. 拉曼光谱测量显示有一1550cm-1为中心的宽峰, 这与用其他方法制备的典型类金刚石碳(DLC)膜相类似, 随着增大氢气压力,膜的吸收系数减小, 而光带隙增大. 在氢气压力为133Pa下沉积的薄膜具有大于2.5eV的光带隙, 比无氢气气氛下沉积的薄膜的光带隙大1倍. 结果表明, 氢气对蚀刻sp2键是有效的. 在PLD法中, 从靶中喷射的等离子体物质和氢分子预先被离解成原子氢, 与CVD法的情况相同,这些原子氢必定起到蚀刻sp2键的作用.     

In Situ FTIR,XPS, and STM Studies of the Nano-Structure of a Photopolymerized C60 Film

Abstract

The nano-structure of C₆₀ photopolymers has been investigated using in situ Fourier-Transform Infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). The FTTR and XPS studies revealed that the C₆₀ pholopolymer has a cross-linking via a [2+2] cycloaddition four-membered ring and formed a 2D rhombohedral structure when the polymerization was saturated. Using STM, we have successfully observed the direct real-space imaging of the C₆₀ dimers and trimers in a C₆₀ monolayer supported on a clean semiconductor surface.     

Porous silicon with embedded metal oxides for gas sensing applications

This paper presents an analysis of the sol deposition process on porous silicon in order to produce highly sensitive gas detectors. Sol solutions were deposited within the dendrite structural pore regions of n-type silicon. The parameters for the structures, built of metal oxides (Fe, Ni, Sn), were analyzed. A morphological study of porous silicon, with embedded metal oxide nanocomposites, was carried out using atomic force microscopy. Cross-sections of porous silicon were examined using a scanning electron microscopy. Impedance spectroscopy was used to study the electrical properties of nanomaterials, based on porous silicon, with embedded metal oxide nanocomposites. Gas-sensitivity measurements of the synthesized samples were made. It turned out that, applying variable frequency interference to the sensory structures in a changing environment, changes the impedance response of gas. New perspectives for increasing sensitivity and selectivity are shown.     

Porous silicon optical filters for biosensing applications

The surface of nanostructured silicon (porous silicon) was biofunctionalized by the deposition of 3-aminopropyltriethoxysilane from solution, leading to high density of amine groups covering the surface which would promote the further immobilization of biomolecules. In addition, porous silicon Bragg reflectors were developed for their use in the visible range. The optical behavior of these structures was previously designed by the use of a computational program, from which the optical constants and thickness of the individual porous silicon layers were determined. The possibility of using these structures as biosensors has been explored, based on the significant changes in the reflectance spectra before and after exposing the porous silicon optical structures to biomolecules. In particular, it is shown that there is a notable shift of the reflectance maximum associated to the Bragg reflector after immobilization of polyclonal mouse antibodies. Thus, the experimental results open the possibility of developing biosensors based on the variation of the position of the optical spectrum of porous silicon based devices.