A study of optical properties of hydrogenated microcrystalline silicon films prepared by plasma enhanced chemical vapor deposition technique at different conditions of excited power and pressure

Two sets of hydrogenated microcrystalline silicon thin-film samples were prepared by Plasma Enhanced Chemical Vapor Deposition (PECVD) technique at different deposition conditions of excited power and pressure. The correlation between the crystalline volume fraction for the samples determined from Raman spectra and the excited power, pressure, absorption coefficient, refractive index and optical energy gap was discussed. The values of optical parameters (refractive index and absorption coefficient), were calculated from the transmission spectra in the range 400–2500 nm. The optical band energy gap and Urbach energy were obtained using the calculated values of absorption coefficients     

Optical properties and crystallinity of hydrogenated nanocrystalline silicon (nc-Si:H) thin films deposited by rf-PECVD

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films prepared in a home-built radio-frequency (rf) plasma enhanced chemical vapour deposition (PECVD) system have been studied. The rf powers were fixed in the range of 5 W-80 W. The optical properties and crystallinity of the films were studied by X-ray diffraction (XRD), Micro-Raman scattering spectroscopy, high resolution transmission electron microscope (HRTEM), and optical transmission and reflection spectroscopy. The XRD and Micro-Raman scattering spectra were used to investigate the evidence of crystallinity in order to determine the crystallite sizes and crystalline volume fraction in the films. The HRTEM image of the film was used to correlate with the crystallinity that was determined from XRD and Micro-Raman scattering spectra. Optical constants such as refractive index, optical energy gap, Tauc slope, Urbach energy and ionic constants were obtained from the optical transmission and reflectance spectra. From the results, it was interesting to found that the optical constants showed a good correlation with the crystallinity within the variation of rf power. Also, the ionic constants of the films showed an indication of the degree of crystallinity in the films. The variation of the optical energy gap with the rf power based on structure disorder and the quantum confinement effect is discussed.     

Influence of bias voltage on the optical and structural properties of nc-Si:H films grown by layer-by-layer (LBL) deposition technique

The effects of applying a positive bias of 25 to 100 V on the optical, structural and photoluminescence (PL) properties of hydrogenated nanocrystalline silicon (nc-Si:H) films produced by layer-by-layer (LBL) deposition technique has been studied. Optical characterization of the films has been obtained from UV-VIS-NIR spectroscopy measurements. Structural characterization has been performed using X-ray diffraction, micro-Raman spectroscopy and field emission scanning electron microscope (FESEM). PL spectroscopy technique has been used to investigate the PL properties of the films. In general, the films formed shows a mixed phase of silicon (Si) nanocrystallites embedded within an amorphous phase of the Si matrix. The crystalline volume fraction and grain size of the Si nanocrystallites have been shown to be strongly dependent on the applied bias voltage. High applied bias voltage enhances the growth rate of the films but reduces the refractive index and the optical energy gap of the films. Higher crystalline volume fraction of the films prepared at low bias voltages exhibits room temperature PL at around 1.8 eV (700 nm).     

Surface plasmon effect in nanocrystalline copper/DLC composite films by electrodeposition technique

Composite films of nanocrystalline copper embedded in DLC matrix prepared by electrodeposition technique were studied for their optical properties. Particle size and metal volume fractions were tailored by varying the amount of copper containing salt in the electrolyte. Blue-shift of the surface plasmon resonance peak in the absorbance spectra of the films was observed with the reduction in size and volume fraction of metal particles. Mie theory was found to describe the experimental spectra quite well.     

铝诱导晶化真空蒸镀多晶硅薄膜的研究

采用真空蒸镀的方法在玻璃衬底上沉积1层非晶硅薄膜,再通过铝诱导晶化的方法制备出晶粒分布较均匀、晶粒尺寸0.5～5μm、晶化率达到89%的多晶硅薄膜。研究了衬底距离、衬底温度、退火温度对薄膜表面形貌、晶粒尺寸和分布及晶化率的影响。结果表明适中的衬底距离下得到的薄膜晶粒分布均匀,表面平整度好,薄膜厚度较大。薄膜的晶化率随着衬底温度和退火温度的提高而增大;随着退火温度的进一步提高,薄膜的晶化率达到最大值然后降低。     

The DC conductivity and structural ordering of thin silicon films at the amorphous to nano-crystalline phase transition

Thin silicon films were deposited by the plasma-enhanced chemical vapor deposition using standard 13.6 MHz radiofrequency gas discharge in silane diluted by hydrogen. The deposition conditions were kept constant for all samples, with the exception of only one parameter: the degree of dilution was varied from low values that produce amorphous layers up to the high dilution that resulted in a high degree of crystalline fraction. The structural properties of the samples were analyzed by Raman and grazing incidence small angle X-ray scattering (GISAXS) while direct current (DC) dark conductivity was measured by standard methods.The ratio of the areas under corresponding transversal optical (TO) phonon peaks in Raman was taken as the ratio between crystal and amorphous volume fraction while the shift of the TO peak position was used for the estimation of the crystal size. By increasing the working gas dilution, the crystalline fraction grew from 0% to 40% and the average individual crystal size increased from 2 to 10 nm.The size of the “particles”, estimated by GISAXS using the Guinier approximation, varied from 2 to 4 nm. For a lower working gas dilution, the “particles” detected by GISAXS were spherically symmetric and showed no difference between near surface and “bulk” of the film. For a higher dilution, the particles became asymmetric and larger closer to the surface, which indicates columnar growth.The DC dark conductivity increased exponentially with the crystalline fraction, except for a very low crystal to amorphous volume ratio where the conductivity was larger, probably due to a better ordering of the amorphous phase in the vicinity of the amorphous to crystalline transition.     

Characterization of microcrystallinity in hydrogenated silicon thin films

The characterization of microcrystallinity in undoped hydrogenated silicon thin films by means of X-ray diffraction and Raman spectroscopy is discussed in detail. We present experimental results for glow discharge films prepared under a wide range of conditions. Microcrystallinity is characterized in terms of crystalline volume fraction and crystallite size, and it is correlated with deposition rate and substrate temperature.A systematic discrepancy appears between the crystalline fractions determined from X-ray diffraction and Raman scattering and is discussed according to two possible origins: the different Raman cross-sections for the microcrystalline and amorphous phases, and the inhomogeneity of crystallinity as a function of thickness. We show that the accuracy can be improved by using fully recrystallized samples as a reference.     

Investigations of the optical absorption spectra of porous silicon layers on the Silicon backing by the nondestructive photoacoustic method

This paper presents a series of experimental photoacoustic spectra of porous silicon layers on the crystalline silicon and their numerical analysis performed in the proposed two layer model. The goal of the analysis was to calculate the optical absorption coefficient spectra of porous silicon from the photoacoustic spectra of the porous silicon layer on the silicon backing. The character of the observed optical absorption band associated with the porous silicon was revealed.     

Modification of the structure and hydrogen content of amorphous hydrogenated silicon films under conditions of femtosecond laser-induced crystallization

We have studied the Raman spectra of initially amorphous hydrogenated silicon (a-Si:H) films upon their exposure to femtosecond laser-radiation pulses with the fluence varied within 30–155 mJ/cm². The distribution of the volume fraction of a crystalline phase over the surface of processed films is determined for the first time and a correlation is established between changes in this value and the hydrogen content in a-Si:H films upon the crystallization induced by femtosecond laser radiation.     

In situ micro-Raman spectroscopic study of laser-induced crystallization of amorphous silicon thin films on aluminum-doped zinc oxide substrate

The effects of laser irradiation condition and deposition substrate on the laser crystallized 330?nm Si films structure were investigated using in situ micro-Raman spectroscopy. Results showed that crystallization of amorphous silicon (a-Si) films started at laser irradiation power density of 0.6?×?10⁵?W/cm² for 20?s. The crystalline volume fraction of Si films depended mostly on the laser power density but not on the laser irradiation time. The Si films on both smooth and textured aluminum-doped zinc oxide (AZO) substrates exhibited lower crystalline volume fraction and smaller average grain size than the Si films on glass did. The Si films on textured AZO revealed higher crystalline volume fraction and larger average grain size than the Si films on smooth AZO did. The stress in crystalline Si films was observed to be compressive on AZO and tensile on glass. The compressive stress in crystalline Si films on textured AZO was slightly less than that on smooth AZO. The present work indicated that the structure of crystalline Si films on textured AZO was improved than that on smooth AZO, which may be helpful to crystalline Si thin-film solar cell.     

Effects of low argon dilution ratio on the nanocrystallization and properties of a-Si:H thin films

The structure and properties of thin films obtained by radio frequency (RF) plasma-enhanced chemical vapor deposition (PECVD) through decomposition of silane mixed with argon have been studied by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, ultraviolet and visible (UV-vis) spectroscopy and transmission electron microscope (TEM), respectively. The dilution ratio increases from 6 to 24, and other deposition parameters are fixed. It is observed that argon as dilution gas in the deposition process benefits the development of nano grains and the growth of thin films. The crystalline volume fraction, grain size and hydrogen content of the films increase with the increase argon dilution ratio. The dark conductivity of the thin films is measured, and the results show that the increase of crystalline volume fraction improves the electrical property of the films. The optical gap of the films is calculated from details of the spectra measured by UV-vis. The decrease of optical gap of the films with the increase of argon dilution ratio has been observed.     

射频功率对微晶硅薄膜微结构和光电性能的影响

采用射频等离子体增强化学气相沉积(rf-PECVD)技术,在玻璃和硅衬底上沉积微晶硅(μc-Si:H)薄膜。利用拉曼光谱、AFM和电导率测试对不同射频功率下沉积的薄膜的结构特性及光电性能进行分析。研究表明:随着射频功率的增加,薄膜的晶化率和沉积速率也随之增加,而当射频功率增加到一定的程度,晶化率和沉积速率反而减小。薄膜的暗电导率与晶化率的变化情况相对应。     

Structural characterization of boron doped hydrogenated nanocrystalline silicon films

Structural properties of boron doped hydrogenated nanocrystalline silicon films deposited by plasma enhanced chemical vapor deposition method were mainly characterized with Raman and X-ray diffraction methods. The experimental Raman data were fitted better by Fano effect profiles than those by phonon confinement effect line shapes chiefly due to high efficiency doping in grown films. The measured Raman spectra were deconvoluted into three-Gaussian profile components: around the peak positions 520 and 480 cm⁻¹ which contribute from crystalline and amorphous tissues separately, as well as a curve centered at about 500 cm⁻¹, which is attributed to the presence of grain boundaries. The average crystalline grain size and crystalline volume fraction were valued with Raman and X-ray diffraction techniques, respectively, while the error derived from different methods was elucidated. Accordingly, the structural changes including crystallites, grain boundaries and amorphous matrices in doped films with boron doping level were analyzed.     

Structural and optical properties of nc-Si:H thin films deposited by layer-by-layer technique

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films deposited on c-Si and quartz substrates by layer-by-layer (LBL) technique using radio-frequency plasma enhanced chemical vapour deposition system. The effects of rf power on the interlayer elemental profiling, structural and optical properties of the films were investigated by Auger electron spectroscopy, Fourier transform infrared spectroscopy, Raman scattering spectroscopy, X-ray diffraction and optical transmission and reflection spectroscopy. The results revealed that the LBL deposition leads to a formation of different ranges of crystallite sizes of nc-Si corresponds 3–6 and 8–26 nm respectively. LBL deposition also demonstrated a capability to increase the crystalline volume fraction of nc-Si up to 65.3 % with the crystallite size in between 5 and 6 nm, at the rf power in between 80 and 100 W. However, the crystalline volume fraction decreased for the rf power above 100 W due to the growth of nc-Si was suppressed by the formation of SiO₂. In addition, the onset of crystallization of the films deposited on c-Si and quartz substrates are different with increase in the rf power. The effects of rf power on the growth of nc-Si, and the hydrogen content, structural disorder, crystallite size of nc-Si and oxygen diffusion into the LBL layer with the change of optical energy gap under the variation of rf power are also discussed.     

氮对纳米硅氮薄膜晶化的影响

在电容式耦合等离子体化学气相沉积系统中，用高氢稀释硅烷和氮气为反应气氛制备纳米硅氮（ｎｃ－ＳｉＮｘ２Ｈ）薄膜，结果表明，当Ｎ２／ＳｉＨ４气体流量比（Ｘｎ）从Ｉ增加为４时，薄膜的晶态率从５８％降至１４％，晶粒尺寸从１０ｎｍ降至５ｎｍ，Ｎ／Ｓｉ含量比从０．０３增至０．１２，当Ｘｎ≥５，则生成非晶硅氮（ａ－ＳｉｕＮｘ２Ｈ）薄膜，当Ｘｎ从１增加为１０时，薄膜暗电导率从１０＾－５（Ωｃｍ）＾－１降至１０＾－     

Nanocrystalline silicon prepared at high growth rate using helium dilution

Growth and optimization of the nanocrystalline silicon (nc-Si: H) films have been studied by varying the electrical power applied to the helium diluted silane plasma in RF glow discharge. Wide optical gap and conducting intrinsic nanocrystalline silicon network of controlled crystalline volume fraction and oriented crystallographic lattice planes have been obtained at a reasonably high growth rate from helium diluted silane plasma, without using hydrogen. Improving crystallinity in the network comprising ∼ 10 nm Si-nanocrystallites and contributing optical gap widening, conductivity ascending and that obtained during simultaneous escalation of the deposition rate, promises significant technological impact.     

Functionalization control of porous silicon optical structures using reflectance spectra modeling for biosensing applications

Modeling and experimental reflectance spectra of porous silicon single layers at different steps of functionalization and protein grafting process are adjusted in order to determine the volume fraction of the biomolecules attached to the internal pore surface. This method is applied in order to control the efficiency of the chemical functionalization process of porous silicon single layers. Using results from single porous silicon layer study, theoretical microcavity is simulated at each step of the functionalization process. The calculated reflectance spectrum is in good agreement to the experimental one. Therefore the single layers study can be applied to multilayer structures and can be adapted for other optical structures such as waveguides, interferometers for biosensing applications.     

The effect of heat-treatment on the grain-size of nanodisperse plasmathermal silicon nitride powder

Nanodisperse silicon nitride has been synthesized by vapor phase reaction of silicon tetrachloride and ammonia in a thermal plasma reactor and crystallized at temperatures 1250, 1350, 1450 and 1500°C. The average grain-size and the dislocation density of the samples were determined by the recently developed modified Williamson-Hall and Warren-Averbach procedures from X-ray diffraction profiles. A new numerical method provided log-normal grain-size distributions from the size parameters derived from X-ray diffraction profiles. It has been shown that the average grain-size in the amorphous phase is lower than that observed in the crystalline fraction. On the other hand, the average grain-size in the crystalline fraction decreases up to 1450°C while it increases during heat-treatment at 1500°C. The size distribution and the area-weighted average grain-size obtained by X-rays were in good agreement with those determined by TEM and from the specific surface area, respectively. The dislocation density was found to be of the order of 10¹⁴ and 10¹⁵ m^–2.     

Smart optical sensors for chemical substances based on porous silicon technology

A simple geometry optical sensor based on porous silicon technology is theoretically and experimentally studied. We expose some porous silicon optical microcavities with different porous structures to several substances of environmental interest: Very large red shifts in the single transmission peak in the reflectivity spectrum due to changes in the average refractive index are observed. The phenomenon can be ascribed to capillary condensation of vapor phases in the silicon pores. We numerically compute the peak shifts as a function of the liquid volume fraction condensed into the stack by using the Bruggeman theory. The results presented are promising for vapor and liquid detection and identification.     

Microstructure and electronic properties of microcrystalline silicon carbide thin films prepared by hot-wire CVD

An overview on microstructural and electronic properties of stoichiometric microcrystalline silicon carbide (μc-SiC) prepared by Hot-Wire Chemical Vapor Deposition (HWCVD) at low substrate temperatures will be given. The electronic properties are strongly dependent on crystalline phase, local bonding, strain, defects, impurities, etc. Therefore these quantities need to be carefully investigated in order to evaluate their influence and to develop strategies for material improvement. We will particularly address the validity of different experimental methods like Raman spectroscopy and IR spectroscopy to provide information on the crystalline volume fraction by comparing the results with Transmission Electron Microscopy (TEM) and X-Ray diffraction data. Finally the electronic properties as derived from optical absorption and transport measurements will be related to the microstructure.