Crystallization of n-doped amorphous silicon PECVD films: comparison between SPC and RTA methods

Amorphous silicon films are deposited by radio-frequency plasma-enhanced chemical-vapor deposition (RF-PECVD) with different n-doping rates. The amorphous films are subsequently crystallized using either solid phase crystallization (SPC) or rapid thermal annealing (RTA). We compare the effect of the n-doping rate on some properties of the microcrystalline silicon films obtained with both techniques. In the SPC process, the time required for the beginning of the crystallization decreases with increasing phosphorus doping. Moreover, doped films present slightly higher crystal size than intrinsic films but the doping rate does not significantly influence the grain size. For RTA, the doping rate decreases the crystallization temperature and increases significantly the crystal size. Whatever the doping rate, the average grain sizes obtained by RTA are larger than those obtained by SPC. The electrical resistance of the crystallized films also depends on the crystallization process: RTA films present a lower dark conductivity than SPC films. These results are discussed taking into account the different kinetics of both crystallization techniques and the role played by the silicon dangling bonds and their charge states on the crystal growth.     

Peculiarity of constant photocurrent method for silicon films with mixed amorphous–nanocrystalline structure

The results of conductivity, photoconductivity and constant photocurrent method absorption measurements by DC and AC methods in hydrogenated silicon films with mixed amorphous–nanocrystalline structure are presented. A series of diphasic silicon films was deposited by very high frequency plasma enhanced chemical vapor deposition technique, using different hydrogen dilution ratios of silane. The increase of hydrogen dilution ratio results in five orders of magnitude increase of conductivity and a sharp increase of grain volume fraction. The comparison of the absorption spectra obtained by DC and AC methods showed that they are similar for silicon films with the predominantly amorphous structure and films with high grain volume fraction. However we found a dramatic discrepancy between the absorption spectra obtained by DC and AC constant photocurrent methods in silicon films deposited in the regime of the structure transition from amorphous to nanocrystalline state. AC constant photocurrent method gives higher absorption coefficient than DC constant photocurrent method in the photon energy range of 1.2–1.7 eV. This result indicates the possibility of crystalline grains contribution to absorption spectra measured by AC constant photocurrent method in silicon films with intermediate crystalline grain volume fraction.     

Remote plasma deposition of microcrystalline silicon thin-films: Film structure and the role of atomic hydrogen

Microcrystalline silicon films grown in an expanding thermal plasma, i.e. in the absence of ion bombardment, are found to be porous and rich in nano-sized voids. By carrying out an extensive investigation on the material quality of films deposited in the amorphous-to-microcrystalline transition regime, on the microcrystalline silicon growth development, and on the influence of the substrate temperature, it is concluded that the inferior material quality is related to the lack of a sufficient amount of amorphous silicon tissue. As possible cause for the insufficient amount of amorphous silicon tissue, the interaction of atomic hydrogen with amorphous silicon films has been studied in order to highlight a possible competition between film growth and H-induced etching of amorphous silicon, and between film growth and H-induced surface/film modification. The etch rates obtained are too low to compete with film growth. Furthermore, atomic H cannot be considered responsible for the poor quality of amorphous tissue present in the microcrystalline silicon films, as the H up-take mainly takes place in divacancies. These results suggest that ion bombardment may be a necessary condition to provide good quality microcrystalline silicon films.     

单晶硅外延生长晶化硅薄膜的研究

采用P型单晶硅片为衬底,并经混合酸溶液腐蚀抛光、清洗后,利用射频磁控溅射镀膜系统在其表面制备非晶硅薄膜;再结合快速光热退火工艺,于N2气氛下480℃退火30 min,得到晶化硅薄膜;利用光学金相显微镜、XRD衍射仪和拉曼散射光谱(Raman)仪对单晶硅衬底和晶化硅薄膜进行结构和性能表征.研究了混合酸溶液对单晶硅表面腐蚀效果、籽晶诱导外延生长晶化硅薄膜的物相结构和薄膜带隙.结果表明:采用混合酸溶液腐蚀后得到表面平整、光滑的单晶硅衬底;非晶硅薄膜经过快速退火后受籽晶诱导生成晶化硅薄膜,其晶相沿单晶硅衬底取向择优生长;随着非晶硅薄膜厚度从80 nm增加到280 nm,晶化后硅薄膜的表面粗糙度逐渐减小,晶化率从90.0;逐渐降低到37.0;;晶粒尺寸从6.65 nm逐渐减小到1.71 nm;带隙从1.18 eV逐渐升高到1.52 eV.     

An EXAFS study of the local structure of Ce---Si---O amorphous thin films

An EXAFS study has been made on the structure of three composition ranges of Ce---Si---O amorphous thin films prepared by RF sputtering. The measurements, carried out on the K edge of silicon and the L₃ edge of cerium, reveal that in the stoichiometric oxygen films of the general formula (Ce, Si) O₂, both cerium and silicon are four-coordinated by oxygen regardless of the O : Si ratio. In the oxygen-deficient films cerium remains four-coordinated by oxygen, but, around silicon, the oxygen atoms are progressively replaced by silicon as the oxygen content of the films is reduced. In silicon-rich films which are very deficient in oxygen, the oxygen atoms prefer to ramain coordinated with cerium, rather than silicon.

A definite decrease in the Si---O distance with increase in Si---O coordination has been found. The effect is attributed to an increase in the charge of silicon with oxygen coordination, and supports a randomly bonded model for the structure.

The total oxygen coordination, derived from a consideration of bond conservation, indicates that the film structures are probably SiO₂-type continuous random networks.     

Spectral and barrier properties of heterocomposites based on poly (para-phenylene) disposed between the silicon films

ABSTRACT

Within the framework of the methods of the electron density functional and the ab initio pseudopotential, we have obtained the spatial distributions of the valence electrons density, the electron energy spectra and the Coulomb potential for heterocomposites based on poly (para-phenylene) and carbon nanotube disposed between the silicon films.

?It was revealed that the maximal value of the potential barrier was noticed in a composite material from polyparaphenylene filaments placed between silicon films and completed with carbon nanotubes perpendicularly to the surface of the silicon films and along them.     

Reconstruction of the surface layer density profile by the X-ray reflectometry method

The method for reconstructing the profile of the material density distribution arbitrarily varying over the depth of the subsurface layer of the specimen from X-ray reflectometry data is suggested. For the first time, the recurrent relationships for the derivatives of the specular reflection coefficient with respect to the parameters of the subsurface layer are obtained, which reduce the volume of the necessary computations by one to two orders of magnitude. The class of functions for which such density-profile reconstruction can be made within a 5% error is characterized. The method is tested on thin tungsten and carbon films on silicon substrates and thin films of porous silicon films with p ⁺-type conductivity.     

Properties of non‐stoichiometric nitrogen doped LPCVD silicon thin films

The influence of nitrogen on the internal structure and so on the electrical properties of silicon thin films obtained by low‐pressure chemical vapor deposition (LPCVD) was studied using several investigation methods. We found by using Raman spectroscopy and SEM observations that a strong relationship exists between the structural order of the silicon matrix and the nitrogen ratio in film before and after thermal treatment. As a result of the high disorder caused by nitrogen on silicon network during the deposit phase of films, the crystallization phenomena in term of nucleation and crystalline growth were found to depend upon the nitrogen content. Resistivity measurements results show that electrical properties of NIDOS films depend significantly on structural properties. It was appeared that for high nitrogen content, the films tend to acquire an insulator behavior. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low temperature plasma deposition of silicon thin films: From amorphous to crystalline

We report on the epitaxial growth of crystalline silicon films on (100) oriented crystalline silicon substrates by standard plasma enhanced chemical vapor deposition at 175 °C. Such unexpected epitaxial growth is discussed in the context of deposition processes of silicon thin films, based on silicon radicals and nanocrystals. Our results are supported by previous studies on plasma synthesis of silicon nanocrystals and point toward silicon nanocrystals being the most plausible building blocks for such epitaxial growth. The results lay the basis of a new approach for the obtaining of crystalline silicon thin films and open the path for transferring those epitaxial layers from c-Si wafers to low cost foreign substrates.     

The effects of annealing temperature on photoluminescence of silicon nanoparticles embedded in SiNx matrix

The effects of the annealing temperature on photoluminescence (PL) of non-stoichiometric silicon nitride (SiN_x) thin films deposited by plasma enhanced chemical vapor deposition (PECVD) using ammonia and silane mixtures at 200 °C were investigated. The optical property and the chemical composition of the films annealed at different temperatures were investigated by PL spectroscopy and Fourier transform infrared absorption spectroscopy (FTIR), respectively. Based on the PL results and the analyses of the bonding configurations of the films, the light emission is attributed to the quantum confinement effect of the carriers inside silicon nanoparticles and radiative defect-related states. These results provide a better understanding of optical properties of silicon nanoparticles embedded in silicon nitride films and are useful for the application of nanosize silicon semiconductor material.     

Micron scale and nanoscale structure of mesoporous silica thin films

Ordered mesoporous silica thin films have been prepared on silicon substrates by spin-coating technique using poly(alkaline oxide) triblock copolymers EO₂₀PO₇₀EO₂₀ (P123) as structure-directing agent. The X-ray diffraction and transmission electron microscopy investigations show that the obtained mesoporous silica thin films have an ordered pore array structure in nanoscale. The atomic force microscopy analysis reveals that the obtained mesoporous silica thin films exhibit a tile arrangement structure in micron scale.     

PECVD a-Sic:H Young’s modulus obtained by MEMS resonant frequency

In this paper we study the Young’s modulus of PECVD obtained silicon rich (x > 0.5) a-Si_xC_1?x:H thin films through the study of the resonance frequency of free standing cantilevers. These structures are fabricated based on front side bulk micromachining of Si substrate and actuated thermally. In this approach, an alternating electric current passes through a photolithography patterned metallic film deposited on the cantilever, heating the structure by Joule effect and inducing vibrations on the cantilever. This method of actuation is independent of the separation between the structure and substrate, which is its main advantage, because it allows the actuation of cantilevers that are bent upwards or downwards, which is an aspect of particular importance in the characterization of PECVD materials for MEMS applications. The work is focused on low stress silicon rich amorphous hydrogenated silicon carbide films obtained by PECVD at low temperatures (320 °C). The measurements were carried out in groups of cantilevers with different length (between 550 and 200 μm) and utilizing a-SiC:H films obtained with three different compositions. The results show that the films exhibit modulus of elasticity in the range of 20–35 GPa, low residual stress (～90 GPa) and maintain excellent chemical inertness in KOH and HF solutions.     

X-ray diffraction study of gadolinium stearate Langmuir-Blodgett films

The comparative structural studies of gadolinium stearate Langmuir-Blodgett films prepared with the use of two different subphases (aqueous solutions of gadolinium chloride and gadolinium acetate) have been performed by the methods of X-ray diffractometry. The films were applied onto single-crystal silicon substrates coated with a native-oxide layer either with the use of buffer molecular stearic-acid layers or without such layers. It was established that the films obtained with the use of gadolinium acetate and the preliminarily formed buffer layers are, in fact, single-phase Y-type films with highly ordered molecular structure, and the thickness of the bimolecular layer d = (51.3 ± 0.5) Å.     

Variation of crystallization mechanisms in flash-lamp-irradiated amorphous silicon films

Flash lamp annealing (FLA) can form polycrystalline silicon (poly-Si) films with various microstructures depending on the thickness of precursor amorphous Si (a-Si) films due to the variation of crystallization mechanisms. Intermittent explosive crystallization (EC) takes place in precursor a-Si films thicker than approximately 2 μm, and the periodicity of microstructure formed resulting from the intermittent EC is independent of the thickness of a-Si films if their thickness is 2 μm or greater. In addition to the intermittent EC, continuous EC and homogeneous solid-phase crystallization (SPC) also occur in thinner films. These crystallization mechanisms are governed by the ignition of EC at Si film edges and the homogeneous heating of interior a-Si. The results obtained in this study could be applied to control the microstructures of flash-lamp-crystallized poly-Si films.     

Excimer laser crystallization of microcrystalline silicon for TFTs on flexible substrate

Microcrystalline silicon (μc-Si) films have been deposited on PDMS as well as on PEN substrate. Excimer laser annealing was used to improve the crystalline structure and so to obtain high mobility TFTs. The effect of the laser annealing on the crystalline structure of silicon films is studied using different characterization techniques and discussed. Mobility values of 60 cm²/V s with PDMS and 46 cm²/V s with PEN are obtained.     

PECVD法制备不同衬底微晶硅薄膜的研究

采用等离子体增强化学气相沉积(PECVD)法分别在玻璃衬底和p型薄膜硅衬底上制备了微晶硅薄膜。使用拉曼谱仪、紫外-可见分光光度计、傅里叶红外光谱仪等对微晶硅薄膜进行检测,重点研究了硅烷浓度、衬底温度对薄膜沉积速率和晶化率的影响。实验结果表明:两种衬底上薄膜的沉积速率均随硅烷浓度的增大、衬底温度的升高而变大。硅烷浓度对两种衬底的薄膜晶化率影响规律相同,即均随其升高而降低;但两种衬底的衬底温度影响规律存在差别:对玻璃衬底而言,温度升高,样品晶化率减小;而p型薄膜硅衬底则在温度升高时,样品晶化率先增大后减小。此外还发现,晶化率与薄膜光学性能及含氧量存在较密切关联。     

Simulation of temperature and gas density field distribution in diamond films growth on silicon wafer by hot filament CVD

In the present study, the temperature and gas density field inside the hot filament chemical vapor deposition (HFCVD) reactor, which play a determinate role on the growth rate and quality of as-deposited diamond films, are simulated using the finite volume method, and the influence of the size and arrangement of filaments and inlets are investigated. Firstly, the correctness of the simulation model is verified by comparing the temperature data obtained from the simulation with that measured in an actual depositing process, and the results show that the error between them is less than 3%. Thereafter, the deposition parameters are optimized using this model as N(filament number)=6, r(filament radius)=0.4 mm, D(filament separation)=16–18 mm, H(substrate–filament distance)=8–9 mm, and 25 inlets. Finally, diamond films are deposited on silicon (100) wafers using above parameters and the results of characterization by SEM and Raman spectrum exhibit that the deposited diamond films appear homogeneous surface with fine-faceted crystals.     

Metastable effects in silicon thin films: Atmospheric adsorption and light-induced degradation

The effects of exposure to atmosphere (ageing) and light-soaking on coplanar dark- and photo-conductivity of silicon films of varying crystallinity are examined. Dark conductivity generally increases on ageing in films with significant amorphous fraction and decreases in largely crystalline films, and may be reversed by annealing under vacuum at 130 °C consistent with adsorption and desorption of atmospheric components. Thinner films are more strongly affected by ageing. Boron doping appears to compensate charge introduced by ageing, though there are disagreements in detail. In comparison with ageing, moderate light-soaking affects dark conductivity in transitional microcrystalline silicon films only slightly. Both processes change the majority carrier mu–tau product in line with shifts in Fermi level position.     

AMPS-1D simulation studies of electronic transport in n+-μc-Si:H thin films

To study the electronic transport in highly n-doped microcrystalline silicon (n⁺-μc-Si:H) thin films, grain-boundary trapping model is implemented in AMPS (analysis of microelectronic and photonic structure)-1D. This approach is based on the traditional thermionic-emission model and considering the electronic transport parallel to the substrate. In spite of its simplicity, the model leads to the simulated values of activation energy, free carrier concentration, interface trap charge density and mobility which are in good agreement with the referred Hall effect measurement results for electron cyclotron resonance-chemical vapor deposited (ECR-CVD) highly n-doped μc-Si:H thin films.     

Synthesis of polycrystalline silicon thin films with ‘icosahedral’ symmetry by ceramics hot wire chemical vapor deposition

We report on synthesis and materials physics of polycrystalline silicon thin films deposited on glass with rarely observed ‘five-fold’ symmetry or ‘icosahedral’ symmetry. We invented these ‘novel form’ of polycrystalline silicon thin films by ceramics hot wire chemical vapor deposition (hot-wire CVD). A new physical effect in hot-wire CVD technology has been proposed that controls the nucleation and growth of silicon thin films on glass substrate.