High-temperature annealing of macroporous silicon in an inert-gas flow

Interest in the sintering of macroporous silicon is due to the possibility of purposefully modifying its structure. The annealing of macroporous structures in an atmosphere of Ar, instead of H₂, simplifies the requirements to equipment and safety engineering. The sintering of macroporous silicon as result of annealing at T = 1000–1280°C in a horizontal tube purged with high-purity gases: Ar or Ar + 3%H₂ is examined. Experiments were conducted with layers having deep cylindrical macropores produced by the electrochemical etching of samples with seed pits on their surface (ordered pores) and without seeds (random pores). The morphology of the porous structure and the changes in this structure upon annealing are studied with electron and optical microscopes. It is shown that, depending on the pore diameter and treatment temperature, the following transformation occurs: the pore surface is smoothed, pores are closed and a surface crust is formed, cylindrical pores are spheroidized and decompose into isolated hollow spheres, and a fine structure and faceting are formed. It is shown that the (111) planes have the minimal surface energy. It is found that the annealing of macroporous silicon in an inert gas leads to strong thermal etching, which is manifested in the fact that the porosity increases or even the porous layer at the sample edge fully disappears. Moreover, an oxide layer appears as a film, beads, or long filaments forming a glass wool upon annealing, especially at low temperatures. These features can be attributed to the presence of trace amounts of an oxidizing agent in the inert gas, which causes the formation of highly volatile SiO and products formed in the reaction involving this compound.     

Oxidative-gravimetric porosimetry of macroporous silicon

A simple nondestructive method to find the internal surface area, porosity, pore diameter, and pore density in macroporous silicon with through channels is suggested and tested. The porosity p is determined from the mass loss upon anodizing, and the surface area per unit volume, S _v , from the mass of SiO₂ formed on the pore surface upon thermal oxidation. The relations are given for calculation of the average pore diameter d and pore density N from the obtained S _v and p. Dependences of the specific surface area and porosity on the resistivity of initial n-Si in the range ρ = 3–25 Θ cm have been studied for samples with ordered and self-organized “lattices” of macropores. The obtained values are within the limits p = 27–50%, S _v = 2800–6000 cm²/cm³, d = 1.9–6.5 μm, and N = 1.4?10 × 10⁶ cm^?2, in agreement with the data fumished by microscopy.     

Growth of silicon nanoclusters in thermal silicon dioxide under annealing in an atmosphere of nitrogen

It is found for the first time that silicon nanoclusters are formed in the surface layer of thermal silicon dioxide under high-temperature annealing (T = 1150°C) in dried nitrogen. Analysis of the cathodoluminescence spectra shows that an imperfect surface layer appears upon such annealing of silicon dioxide, with silicon nanoclusters formed in this layer upon prolonged annealing. Transmission electron microscopy demonstrated that the silicon clusters are 3–5.5 nm in size and lie at a depth of about 10 nm from the surface. Silicon from the thermal film of silicon dioxide serves as the material from which the silicon nanoclusters are formed. This method of silicon-nanocluster formation is suggested for the first time.     

Preparation and degradation of the optical properties of nano-, meso-, and macroporous silicon

In the work some peculiarities in formation of the porous silicon, its morphology and composition were investigated, as well as some optical properties of this material. Porous silicon was obtained on the substrates of single-crystalline Si as well as on the silicon structures with p–n junctions. In order to obtain nano-, meso- and macroporous silicon as well as multi-layered porous structures several technological parameters (were controlled) varied—orientation of the substrates, conductivity type and composition of the etchant. Correlation between photoluminescence intensity of the obtained samples and intensity of the absorption band in their IR spectra (the band at 616 cm⁻¹) that is due to the presence of Si–Si bonds in porous layer was observed. The influence of the porous silicon storage in the air on the degradation of its photoluminescence parameters was also estimated.     

Optical transparency of macroporous silicon with through pores

Optical transmission spectra of macroporous silicon with through pores has been studied experimentally and theoretically in the spectral range 0.3–0.8 μm (in which single-crystal Si is opaque). The spectra are discussed in terms of the general diffraction theory. Expressions taking into account the diffraction at the entrance and exit of pores and the influence exerted by deviation of the angle of light incidence from the normal are derived. The transmission spectrum is fairly well described by the results of calculation. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 1, 2003, pp. 57–64. Original Russian Text Copyright ? 2003 by Astrova, Korovin, Lang, Remenyuk, Shuman.     

硅振荡器特性分析研究

为了研究硅振荡器的特性,本文采用硅振荡器和传统石英晶振的对比实验来研究,通过对石英晶振的特性比较全面的把握,利用石英晶振的频率值精度高、温漂小、抖动不明显的特征,来研究硅振荡器的实际频率值、温漂和抖动特性,所采用的方法是使用鉴相器的频率合成方法来比对得出硅振荡器的特性参数,其中选用MAX7375硅振荡器和KSS石英晶振通过温度的变化以及抖动的测试完成硅振荡器的初步特性分析,方便为后期使用硅振荡器来校准晶振做基础性研究。     

Strains and crystal lattice defects arising in macroporous silicon under oxidation

The deformation behavior of a macroporous silicon wafer subjected to high-temperature oxidation has been studied, and the basic parameters describing the sample bending and subsequent stress relaxation when oxide is removed are determined. X-ray diffractometry and topography were used to determine the sample bending radius and lattice parameters, and to reveal the areas of dislocation generation. The strain of a silicon lattice in oxidized macroporous Si is about 10^?4, and it decreases by an order of magnitude after oxide dissolution. The plastic part of the strain is accompanied by the generation of dislocations in the most strained regions of a structure, i.e., at the interfaces between the porous layer and substrate in the vertical direction and between the central porous region and the pore-free edge in the horizontal plane. The dislocation density is ～10⁴ cm^?2.     

Novel method of separating macroporous arrays from p-type silicon substrate

This paper presents a novel method to fabricate separated macroporous silicon using a single step of photo-assisted electrochemical etching. The method is applied to fabricate silicon microchannel plates in 100 mm p-type silicon wafers, which can be used as electron multipliers and three-dimensional Li-ion microbatteries. Increasing the backside illumination intensity and decreasing the bias simultaneously can generate additional holes during the electrochemical etching which will create lateral etching at the pore tips. In this way the silicon microchannel can be separated from the substrate when the desired depth is reached, then it can be cut into the desired shape by using a laser cutting machine. Also, the mechanism of lateral etching is proposed.     

Effect of annealing in an atomic-hydrogen atmosphere on the properties of amorphous hydrated silicon films and the parameters of p-i-n structures based on them

The decrease in the density of dangling silicon-silicon bonds in a-Si:H films as a result of annealing in an atomic-hydrogen atmosphere is determined by their density in the initial (nonannealed) film. The change in the total hydrogen density in a-Si:H films, annealed in an atomic-hydrogen atmosphere, is determined by the type of silicon-hydrogen bonds and the impurity content: The hydrogen content can decrease to 1 at. % in the presence of monohydride bonds (2020 cm⁻¹) and no change is observed in the hydrogen content in the presence of oxygen (≲0.1 at. %). A decrease in the defect density as a result of annealing in an atomic-hydrogen atmosphere is observed for all films. The Staebler-Wronski effect — AM-1 irradiation for 10 h — is observed for all films irrespective of the total hydrogen density, the type of silicon-hydrogen bonds, and the presence of oxygen. Fiz. Tekh. Poluprovodn. 32, 620–626 (May 1998)     

电压对N型宏孔硅阵列光电化学腐蚀的影响

研究了电压对宏孔硅阵列光电化学腐蚀的影响. 根据空间电荷区理论、N型硅片在HF酸溶液中I-V扫描曲线、不同电流密度条件下硅阳极氧化反应机理,系统地分析研究了腐蚀电压对宏孔硅阵列形貌的影响.研究结果表明：电压升高使孔型变差,电压降低则盲孔率提高,腐蚀电压恒定、长时间腐蚀孔将逐渐分叉。根据理论分析,优化了工艺参数,制备出高长径比宏孔硅阵列结构,孔深度为317m,方孔边长为3m,长径比为105。     

Influence of voltage on photo-electrochemical etching of n-type macroporous silicon arrays

The influence of voltage on photo-electrochemical etching(PEC) of macroporous silicon arrays(MSA) was researched.According to the theory of the space charge region,I-V scan curves and the reaction mechanism of the n-type silicon anodic oxidation in HF solution under different current densities,the pore morphology influenced by the working voltage were studied and analyzed in detail.The results show that increasing the etching voltage will lead to distortion of the pore morphology,decreasing etching volta...     

Effects of nitrogen and argon as carrier gases and annealing ambients on the physical properties of PECVD silicon nitride

Deposition of memory quality plasma-enhanced chemically vapor deposited silicon nitride and post-deposition modification by annealing using argon and nitrogen for carrier gases and annealing ambients have been studied. Although similar memory performance can be achieved, the deposition power and NH₃/SiH₄ ratio required to do so are functions of the carrier gas used since nitrogen actively participates in the deposition process. Fourier transform infrared, ultraviolet and Auger data reveal that materials deposited with the two gases have similar physical properties although argon films are less dense and lose hydrogen at a higher rate when subjected to annealing. Post-deposition annealing can be used to enhance memory properties of the material provided that the anneal is accomplished at 475°C for 30 minutes. However, enhancement of memory performance by annealling occurs only if the nitride has been deposited using optimum deposition parameters. When annealing is performed in nitrogen. Auger data suggest that nitrogen can be supplied to, or removed from, the material depending on the anneal temperature.     

Improved photovoltaic properties of amorphous silicon thin-film solar cells with an un-doped silicon oxide layer

This paper proposes the use of undoped hydrogenated microcrystalline silicon oxide (μc-SiO_x:H) deposited on the n-μc-Si:H layer of amorphous silicon single-junction superstrate configuration thin-film solar cells produced through 40 MHz very high frequency plasma-enhanced chemical vapor deposition. Raman spectroscopy and optoelectronic analyses of the undoped μc-SiO_x:H thin film revealed that adding a small amount of oxygen into a μc-network results in a low optical absorption, wide band gap, high optical band gap E₀₄, high refractive index, reasonable conductivity, and crystalline volume fraction, which are advantageous properties in solar cells. Compared with a standard cell, the current density–voltage (J–V) characteristics of the cell with an undoped μc-SiO_x:H/n-μc-Si:H structure showed an enhancement in short-circuit current density J_sc from 13.32 to 13.60 mA/cm², and in conversion efficiency from 8.53% to 8.61%. The increased J_sc mechanism can be attributed to an improved light-trapping capability in the long wavelength range between 510 and 660 nm, as demonstrated by the external quantum efficiency.     

Transmission of a microcavity structure in a two-dimensional photonic crystal based on macroporous silicon

Photonic crystals consist of regularly arranged dielectric scatterers of dimensions on a wavelength scale, exhibiting band gaps for photons, analogous to the case of electrons in semiconductors. Using electrochemical pore formation in n-type silicon, we fabricated photonic crystals consisting of air cylinders in silicon. The starting positions of the pores were photolithographically pre-defined to form a hexagonal lattice of a=1.58 μm. The photonic crystal was microstructured to make the photonic lattice accessible for optical characterization. Samples with different filling factors were fabricated to verify the gap map of electric and magnetic modes using Fourier-transform infrared (IR) spectroscopy. The complete band gap could be tuned from 3.3 to 4.3 μm wavelength. We were able to embed defects such as waveguide structures or microcavities by omitting certain pores. We carried out transmission measurements using a tunable mid-IR optical parametric oscillator. The resonance is compared with theoretical expectations.     

Deep centers introduced by argon ion bombardment in n-type silicon

Platinum-silicon Schottky diodes on epitaxial layers have been subjected to argon ion bombardment, prior to metal evaporation, with doses in the 10¹⁴ ions/cm² range and with energies between 1 and 3.5 KeV. By transient capacitance technique four electron traps have been found. The oxygen-vacancy pair (O-V) located at 0.17 eV below the conduction band, the double negatively charged divacancy (V-V⁼) at 0.23 eV and the phosphorous-vacancy pair (P-V) at 0.44 eV are observed. One more unknown level at 0.36 eV which could be oxygen-related is also found.     

Effect of the duration of electrochemical anodization on the microhardness of macroporous silicon

The dependence of the microhardness H of macroporous silicon layers formed on the duration t _an of electrochemical anodization at a constant current density of J = 20 mA/cm² is studied. It is shown that the nonlinear run of the dependence H(t _an) is due to the influence exerted on the microhardness not only by the surface density of pores, but also by the single-crystal base at L < 10h, where L and h are the thickness of the porous layer and the depth of indenter penetration, respectively.     

The effect of argon implantation on the conductivity of boron implanted silicon

Silicon wafers have been implanted with boron (3 × 10¹⁴ or 1 × 10¹⁵ ions cm^?2) and with argon (up to 1 × 10¹⁵ ions cm^?2). The energies were chosen to approximately superimpose the two impurity distributions. After the boron and argon implantations the sheet resistance of each wafer was measured following annealing in nitrogen at temperatures in the range 400–1050°C. The highest dose argon implantation produced an increase in sheet resistance which persisted throughout the entire temperature range. Lower argon doses produced a reduction in sheet resistance for anneal temperatures between 550 and 800°C. The magnitude of the reduction is a function of the boron and argon doses and of the anneal temperatures. The greatest reduction, observed after a 600°C anneal, was by a factor of 5.8. Above 800°C the low dose argon did not affect the sheet resistance.The observed reduction in sheet resistance is expected to lead to an improvement in metal to p-type silicon contacts. A particular application is in the contacts to resistors in fast bipolar logic circuits. As high electrical activity can be obtained at moderate annealing temperatures with combined boron and argon implantations, these implantations can be carried out at a late stage in an integrated circuit process schedule without the danger of additional movement of existing junctions.     

The properties of nickel in silicon

This paper describes the properties of active nickel centers in silicon. In particular, the role of nickel in both increasing and reducing minority carrier lifetime in silicon integrated circuits is described, and an assessment made of its use in this application.     

Structural transformation of macroporous silicon anodes as a result of cyclic lithiation processes

Anodes based on a regular lattice of macroporous silicon with different periods, sizes, and shapes of pore cross sections are studied. The discharge capacity and its degradation during cycling (embedding and extraction of lithium) are examined. Scanning electron microscopy is used to analyze changes in the electrode structure upon the lithiation/delithiation of Si and to evaluate the elemental composition of the porous material. An ex situ morphological analysis of the electrodes demonstrates that, on the whole, the porous structure is preserved upon cycling and the thickness of silicon walls increases. The degree of Si-wall destruction depends on their initial thickness. Estimates show that the electrolyte reduction process mainly occurs according to the two-electron mechanism, with inorganic salts of lithium formed as a result.     

Electrical properties of nickel in silicon

Electrical activity and energy levels as well as diffusion properties of nickel in silicon have not yet been reliably established. In this paper, we investigated the diffusion and the electrical properties of nickel in silicon to confirm that nickel is electrically active and introduces one acceptor and one donor level by combined measurements of Hall coefficient and DLTS, and measurements of the distribution of electrically active nickel in various silicon diodes by DLTS. The former experiments show that bothn- andp- type silicon are compensated by nickel and that nickel introduces an acceptor level ofE _c-0.47 ± 0.04 eV and a donor level ofE _v +0.18 ± 0.02 eV. The concentrations of these two levels are almost identical over the diffusion temperatures from about 800 to 1100° C, indicating that these donor and acceptor levels are due to different charge states of the same nickel center. In the distribution measurements of electrically active nickel in silicon diodes, we inspected how nickel can be observed by DLTS. It was found that the nickel diffusion intop- n junction is rather complicated, the distribution profiles of nickel in the vicinity of thep- n junction being markedly influenced by an additional heating at elevated temperatures after the nickel diffusion. This gives evidence that the difference in silicon devices used in various studies could give rise to different results.