Aluminum-induced crystallization of amorphous silicon films deposited by hot wire chemical vapor deposition on glass substrates

Aluminum-induced crystallization of amorphous silicon films is discussed. Amorphous Si films were deposited by hot wire chemical vapor deposition onto Al coated glass substrates at 430 °C. Complete crystallization of a-Si films was achieved during a-Si deposition by controlling Al and Si layer thicknesses. The grain structure of the poly-Si films formed on glass substrate was evaluated by optical and electron microscopy. Continuous poly-Si films were obtained using Al layers with a thickness of 500 nm or less. The average grain size was found to be 10-15 μm, corresponding to a grain size/thickness ratio greater than 20.     

多晶硅薄膜的铝诱导晶化法制备及其晶粒的择优取向特性

采用铝诱导非晶硅薄膜晶化技术制备了多晶硅薄膜,并研究了多晶硅的成核和生长特性。非晶硅薄膜采用等离子体增强化学气相沉积法制备,其表面沉积铝薄膜后经不同温度的氮氛围退火处理。结果表明,退火后的硅薄膜层与铝层发生置换,所生长的多晶硅颗粒的平均尺寸约为150nm。X射线衍射分析结果揭示,薄膜的晶向显著依赖于退火温度,较低温度下,铝诱导晶化速率较慢,薄膜的优化晶向与非晶硅薄膜中团簇的初始原子排列趋势紧密相关。而较高温度下,铝诱导晶化促使多晶硅(111)择优成核及随后的固相生长。     

Poly-silicon thin films by aluminium induced crystallisation of microcrystalline silicon

Al-induced crystallisation of microcrystalline Si thin films prepared by electron cyclotron resonance plasma-enhanced chemical vapour deposition (ECR-PECVD) on glass and SiO₂ coated Si wafers has been studied. The starting structure was substrate/μc-Si/Al. Annealing this structure in the temperature range 370-520 °C, immediately following deposition of the Al layer, resulted in successful layer exchange and the formation of a substrate/Al+Si layer/poly-Si geometry. The top poly-Si layer exhibited grain sizes generally in the range ∼2-6 μm, although larger grains were also sparsely present. The films did not exhibit any appreciable degree of preferred orientation. The surface roughness was relatively high with a R_a value of ∼20 nm.     

Thin-film polycrystalline silicon solar cells formed by flash lamp annealing of a-Si films

We have fabricated thin-film solar cells using polycrystalline silicon (poly-Si) films formed by flash lamp annealing (FLA) of 4.5-µm-thick amorphous Si (a-Si) films deposited on Cr-coated glass substrates. High-pressure water-vapor annealing (HPWVA) is effective to improve the minority carrier lifetime of poly-Si films up to 10 µs long. Diode and solar cell characteristics can be seen only in the solar cells formed using poly-Si films after HPWVA, indicating the need for defect termination. The actual solar cell operation demonstrated indicates feasibility of using poly-Si films formed through FLA on glass substrates as a thin-film solar cell material.     

Preparation of high quality polycrystalline silicon thin films by aluminum-induced crystallization

High quality polycrystalline silicon (poly-Si) thin films without Si islands were prepared by using aluminum-induced crystallization on glass substrates. Al and amorphous silicon films were deposited by vacuum thermal evaporation and radio frequency magnetron sputtering, respectively. The samples were annealed at 500 °C for 7 h and then Al was removed by wet etching. Scanning electron microscopy shows that there are two layers in the thin films. After the upper layer was peeled off, the lower poly-Si thin film was found to be of high crystalline quality. It presented a Raman peak at 521 cm^− 1 with full width at a half maximum of 5.23 cm^− 1, which is similar to c-Si wafer.     

Hot-wire chemical vapor deposition and characterization of polycrystalline silicon thin films using a two-step growth method

A two-step growth method was proposed to reduce the amorphous incubation layer in the initial growth of polycrystalline silicon (poly-Si) films prepared by hot-wire chemical vapor deposition (HWCVD). In the two-step growth process, a thin seed layer was first grown on the glass substrate under high hydrogen dilution ratios (φ ≥ 0.9), and then a thick overlayer was subsequently deposited upon the seed layer at a lower φ value. The effect of various deposition parameters on the structural properties of poly-Si films was investigated by Raman spectroscopy and transmission electron microscopy. Moreover, the electrical properties, such as dark and photo conductivities, of poly-Si films were also measured. It was found that the Si incubation layer could be suppressed greatly in the initial growth of poly-Si with the two-step growth method. In the subsequent poly-Si film thickening, a lower φ value of the reactant gases can be applied to enhance the deposition rate. Therefore, a high-quality poly-Si film can be fabricated via a two-step growth method with a sufficient growth rate using HWCVD.     

金属镍诱导p型多晶硅薄膜的光电性能研究

利用电子束蒸镀方法及重掺杂p型硅为蒸发源在K8玻璃衬底上沉积非晶硅薄膜,采用镍诱导晶化法在氮气氛围下进行退火处理制备出p型多晶硅薄膜.研究了不同温度热处理条件对p型多晶硅薄膜的光电性能的影响,通过霍尔测量、拉曼光谱、原子力显微镜、紫外-可见光吸收光谱等测试手段对薄膜进行分析.结果表明,随着晶化温度的提高晶化程度先增强后...     

Structure of polycrystalline silicon films deposited at low temperature by plasma CVD on substrates exposed to different plasma

Polycrystalline silicon (poly-Si) films were deposited on glass substrates (corning 7059) at 300°C by a plasma enhanced chemical vapor deposition (PECVD) from a SiH₄/SiF₄ mixture. All poly-Si films were prepared under the same deposition conditions on the substrates subjected to nitrogen, hydrogen and/or CF₄ plasma with different gas pressures, just before deposition of the poly-Si films. Effects of such pretreatments for substrates on the structural properties of the resultant poly-Si films have been investigated. The Si film deposited on the substrates without any pretreatments was amorphous. However, formation of a strong 〈110〉 preferentially oriented poly-Si with improved crystallinity was obtained for the films deposited on the glass substrate after plasma pretreatments, which exhibit smoother surfaces. This result was interpreted in terms of a removal of weak Si–Si bonds during nucleation and the subsequent grain growth.     

Effect on thickness of Al layer in poly-crystalline Si thin films using aluminum(Al) induced crystallization method

The polycrystalline silicon (poly-Si) thin films were prepared by aluminum induced crystallization. Aluminum (Al) and amorphous silicon (a-Si) layers were deposited using DC sputtering and plasma enhanced chemical vapor deposition method, respectively. For the whole process Al properties of bi-layers can be one of the important factors. In this paper we investigated the structural and electrical properties of poly-crystalline Si thin films with a variation of Al thickness through simple annealing process. All samples showed the polycrystalline phase corresponding to (111), (311) and (400) orientation. Process time, defined as the time required to reach 95% of crystalline fraction, was within 60 min and Al(200 nm)/a-Si(400 nm) structure of bi-layer showed the fast response for the poly-Si films. The conditions with a variation of Al thickness were executed in preparing the continuous poly-Si films for solar cell application.     

Polycrystalline silicon films with nanometer-sized dense fine grains formed by flash-lamp-induced crystallization

Flash lamp annealing (FLA) with millisecond-order pulse duration can crystallize microm-order-thick a-Si films on glass substrates through explosive crystallization (EC), and flash-lamp-crystallized (FLC) poly-Si films consist of densely-packed nanometer-sized fine grains. We investigate the impact of the hydrogen concentration and the defect density of precursor a-Si films on crystallization mechanism and the microstructures of FLC poly-Si films, by comparing chemical-vapor-deposited (CVD) and sputtered precursor a-Si films. Transmission electron microscopy (TEM) observation reveals that FLC poly-Si films with similar periodic microstructures are formed by the FLA of the two kinds of precursor films, meaning no significant influence of hydrogen atoms and defect density on crystallization mechanism. This high flexibility of the properties of precursor a-Si films would contribute to a wide process window to reproducibly form FLC poly-Si films with the particular periodic microstructures.     

Crystallization of amorphous Si thin films by the reaction of MoO3/Al nanoengineered thermite

In this work, we investigated a new crystallization method for amorphous silicon (a-Si) using a mixture of nano-energetic materials: molybdenum oxide and aluminum (MoO₃/Al). The purpose of using nano-energetic materials is to improve the performance of a-Si films with a self-propagating exothermic reaction over a period of microseconds without any substrate damage. The mixture of MoO₃/Al nanopowders was used for a thermite reaction for crystallization of a-Si thin films.Characterization results showed that a-Si thin films were successfully crystallized to poly-Si as evidenced by a Raman peak near 519 cm^− 1. The crystalline volume fraction of poly-Si after the nanoengineered thermite reaction was about 94.7% and poly-Si grains was uniformly distributed with an average grain size of around 40-50 nm. These results indicate that high quality poly-Si thin films were successfully prepared on the substrate.     

Direct growth of large grain polycrystalline silicon films on aluminum-induced crystallization seed layer using hot-wire chemical vapor deposition

Large grain polycrystalline silicon (poly-Si) films on glass substrates have been deposited on an aluminum-induced crystallization (AIC) seed layer using hot-wire chemical vapor deposition (HWCVD). A poly-Si seed layer was first formed by the AIC process and a thicker poly-Si film was subsequently deposited upon the seed layer using HWCVD. The effects of AIC annealing parameters on the structural and electrical properties of the poly-Si seed layers were characterized by Raman scattering spectroscopy, field-emission scanning electron microscopy, and Hall measurements. It was found that the crystallinity of seed layer was enhanced with increasing the annealing duration and temperature. The poly-Si seed layer formed at optimum annealing parameters can reach a grain size of 700 nm, hole concentration of 3.5 × 10¹⁸ cm^− 3, and Hall mobility of 22 cm²/Vs. After forming the seed layer, poly-Si films with good crystalline quality and high growth rate (> 1 nm/s) can be obtained using HWCVD. These results indicated that the HWCVD-deposited poly-Si film on an AIC seed layer could be a promising candidate for thin-film Si photovoltaic applications.     

Preparation of poly-Si films by Cat-CVD for thin film transistor

Polycrystalline silicon (poly-Si) films have been deposited on low-temperature substrates using a hot-wall type catalytic chemical vapor deposition apparatus. Average grain size of the deposited poly-Si films was 10–20 nm. Hall mobility of 2–5 cm² V⁻¹ s⁻¹ was obtained even for a sample left at ambient conditions for a month. The influence of the sidewall temperature on poly-Si film properties has been investigated. The poly-Si films have been prepared under the hot/cold-wall conditions. Comparing crystalline fractions of both films measured by Raman spectroscopy, the difference was small. The crystalline fractions of 89 and 85% were obtained for the hot- and cold-wall conditions, respectively. As for the results of attenuated total reflection Fourier-transform infrared spectroscopy, a distinct difference between the two films was found, H and O atoms were more incorporated in the films deposited under the cold-wall conditions than in the films under the hot-wall conditions.     

Polycrystalline silicon films fabricated by rapid thermal annealing

Poly-crystalline silicon (poly-Si) films were fabricated by rapid thermal annealing (RTA) of amorphous silicon films which were deposited on quartz by hot wire chemical vapor deposition. An insertion of Cr layer can significantly suppress the peeling of Si films during the RTA process. The effect of RTA parameters on the structural properties of poly-Si films was investigated by Raman spectroscopy, X-ray diffraction and scanning electron microscopy. The results show that the crystallinity of the poly-Si films is increased with the increase of RTA temperature and duration. A sharp peak at about 520?cm^?1 is observed in the Raman spectra of poly-Si films annealed at 900 and 1,100?°C for 15?s indicating the excellent crystallinity of the poly-Si films fabricated by RTA. Poly-Si films with high crystalline fraction of 97.3?% were obtained by RTA at 1,100?°C for 20?s.     

Flash-lamp-crystallized polycrystalline silicon films with high hydrogen concentration formed from Cat-CVD a-Si films

We investigate residual forms of hydrogen (H) atoms such as bonding configuration in poly-crystalline silicon (poly-Si) films formed by the flash-lamp-induced crystallization of catalytic chemical vapor deposited (Cat-CVD) a-Si films. Raman spectroscopy reveals that at least part of H atoms in flash-lamp-crystallized (FLC) poly-Si films form Si-H₂ bonds as well as Si-H bonds with Si atoms even using Si-H-rich Cat-CVD a-Si films, which indicates the rearrangement of H atoms during crystallization. The peak desorption temperature during thermal desorption spectroscopy (TDS) is as high as 900 °C, similar to the reported value for bulk poly-Si.     

Influence of laser wavelength and beam profile on Nd:YAG laser assisted formation of polycrystalline-Si films

Influence of wavelengths and beam profiles of a pulsed Nd³⁺:YAG laser on the formation of a polycrystalline-silicon (poly-Si) on a-Si thin film is investigated. Two sets of samples of amorphous-Silicon (a-Si) thin films deposited on glass (a-Si/glass) and crystalline Si (a-Si/c-Si) substrates were treated with different laser-fluence values. After the laser treatment, the films were analyzed by a scanning electron microscope, the Raman spectroscopy technique and the resistance-measurement technique. In the case of the third harmonics (355 nm) of the Nd³⁺:YAG laser, poly-Si films were obtained with laser-fluence values ranging from 260 mJ/cm² to 560 mJ/cm², where as in the case of the second harmonics (532 nm), the process window for the formation of poly-Si films, in terms of the laser fluence, was ranging from 300 mJ/cm² to 480 mJ/cm². On the other hand, in the case of samples treated with the fundamental wavelength (1064 nm), a narrow process window with higher laser-fluence values around 1100 mJ/cm² was observed. Further, the substrate was also affected because of the higher laser-fluence value. It has also been observed that the crystallization characteristics of poly-Si films improved with the flat-top intensity distribution as compared to the Gaussian intensity distribution of the Nd³⁺:YAG laser beam. A theoretical simulation based on thermal modeling was performed to understand the mechanism of crystallization.     

Precursor Cat-CVD a-Si films for the formation of high-quality poly-Si films on glass substrates by flash lamp annealing

Amorphous Si (a-Si) films with lower hydrogen contents show better adhesion to glass during flash lamp annealing (FLA). The 2.0 µm-thick a-Si films deposited by plasma-enhanced chemical vapor deposition (PECVD), containing 10% hydrogen, start to peel off even at a lamp irradiance lower than that required for crystallization, whereas a-Si films deposited by catalytic CVD (Cat-CVD) partially adhere even after crystallization. Dehydrogenated Cat-CVD a-Si films show much better adhesion to glass, and are converted to polycrystalline Si (poly-Si) without serious peeling, but are accompanied by the generation of crack-like structures. These facts demonstrate the superiority of as-deposited Cat-CVD a-Si films as a precursor material for micrometer-thick poly-Si formed by FLA.     

Influence of annealing temperature on the properties of polycrystalline silicon films formed by rapid thermal annealing of a-Si:H films

In this work, rapid thermal annealing (RTA) was employed to crystallize the amorphous silicon films deposited by hot-wire chemical vapor deposition. The influence of annealing temperature on structural and electrical properties was studied by Raman spectroscopy, X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy and temperature-dependent conductivity measurement. The results show that the amorphous silicon films can be successfully crystallized by RTA in a very short time. The crystallinity and electrical properties of the poly-Si films was greatly improved as the RTA temperature increasing. When the temperature higher than 900 °C, the poly-Si films obtained the crystalline fraction above 95 %, and the hydrogen atoms almost disappeared in the poly-Si films. At the temperature of 1,100 °C, polycrystalline silicon films with conductivity of 16.4 S cm^?1 is obtained, which is seven orders in magnitude higher than that of the film annealed at 700 °C.     

Defect annealing processes for polycrystalline silicon thin-film solar cells

A variety of defect healing methods was analyzed for optimization of polycrystalline silicon (poly-Si) thin-film solar cells on glass. The films were fabricated by solid phase crystallization of amorphous silicon deposited either by plasma enhanced chemical vapor deposition (PECVD) or by electron-beam evaporation (EBE). Three different rapid thermal processing (RTP) set-ups were compared: A conventional rapid thermal annealing oven, a dual wavelength laser annealing system and a movable two sided halogen lamp oven. The two latter processes utilize focused energy input for reducing the thermal load introduced into the glass substrates and thus lead to less deformation and impurity diffusion. Analysis of the structural and electrical properties of the poly-Si thin films was performed by Suns-V_OC measurements and Raman spectroscopy. 1 cm² cells were prepared for a selection of samples and characterized by I–V-measurements. The poly-Si material quality could be extremely enhanced, resulting in increase of the open circuit voltages from about 100 mV (EBE) and 170 mV (PECVD) in the untreated case up to 480 mV after processing.     

Highly conducting doped poly-Si deposited by hot wire CVD and its applicability as gate material for CMOS devices

Highly conducting p- and n-type poly-Si:H films were deposited by hot wire chemical vapor deposition (HWCVD) using SiH₄+H₂+B₂H₆ and SiH₄+H₂+PH₃ gas mixtures, respectively. Conductivity of 1.2×10² (Ω cm)⁻¹ for the p-type films and 2.25×10² (Ω cm)⁻¹ for the n-type films was obtained. These are the highest values obtained so far by this technique. The increase in conductivity with substrate temperature (T_s) is attributed to the increase in grain size as reflected in the atomic force microscopy results. Interestingly conductivity of n-type films is higher than the p-type films deposited at the same T_s. To test the applicability of these films as gate contact Al/poly-Si/SiO₂/Si capacitor structures with oxide thickness of 4 nm were fabricated on n-type c-Si wafers. Sputter etching of the poly-Si was optimized in order to fabricate the devices. The performance of the HWCVD poly-Si as gate material was monitored using C–V measurements on a MOS test device at different frequencies. The results reveal that as deposited poly-Si without annealing shows low series resistance.