Study On Low Temperature Rapid Crystallization Of Aluminum - Induced Amorphous Silicon Films

Amorphous silicon(a-Si) as one of the premier materials for silicon-based thin films has large absorption coefficients, good performance under low illumination, and lower material usage(only 1%) to conventional crystalline silicon in solar cell applications. However, a-Si thin film solar cells are limited by low conversion efficiencies and the light-induced recession effect(the S-W Effect), etc. In contrast, crystallized silicon thin films, including polysilicon,microcrystalline silicon and nano-crystalline silicon thin films have good photoelectric performance but less severe light-induced recession, etc. In addition, the photoelectric performance of crystallized silicon films can be controlled by the parameters of crystallization rate and grain size. Therefore, they become one of the ideal photovoltaic materials.In this paper, a magnetron sputtering system and a rapid photothermal annealing furnace were used to prepare silicon films to study the aluminum induced rapid crystallization of amorphous silicon films at low temperatures. The optimal process parameters for the crystallized silicon thin films were obtained by analyzing their properties. Through the calculation of the crystallization rate, the effect of aluminum-induced crystallization was interpreted and the relationships between the band gap, grain size and crystallization rate were revealed.The following research achievements have been accomplished in this paper:1. The silicon films were prepared at different substrate temperatures and sputtering powers.The effects of the substrate temperature and the sputtering power on the phase structure,thickness uniformity, deposition rate and optical properties of the silicon films were researched. The SPSS statistical analysis shows that the films prepared in our experimental process were all amorphous silicon. The deposition rate, uniformity and optical properties were first increased and then decreased with increasing substrate temperature and sputtering power. The optimal process was obtained at the substrate temperature of 200 ℃ and the power of 100 W.2. The influences on the aluminum induced crystallization of amorphous silicon films by using different substrates and substrate temperatures were studied. The silicon films were prepared in the structures of glass/α-Si/Al and Si(400)/α-Si/Al and annealed at 400 ℃ for 25 min. The films deposited on glass substrates show crystallized silicon peaks of the(111) and(220) planes. While those on Si substrates only exhibits the(400) peak, which is in consistent with the substrate orientation. As a result, films prepared on single crystalline silicon substrates show an improved crystallization than those on glass substrates. Increasing the substrate temperature also improves the crystallization rate and promotes the grain size growth. However, the crystallization rate and grain size are decreased when the temperature is too high. The optimal substrate temperature is 200 ℃.3. The magnetron sputtering coating system was used to prepare amorphous silicon thin films with different thicknesses on c-Si substrates. The deposited films were annealed at 400 ℃and 25 min in N2 to achieve crystallization. The influence of monocrystalline silicon substrate on the crystallization of amorphous silicon films was studied. The results show that crystallized silicon films were prepared after annealing and the preferential crystalline film growth orientation was along the seed crystal orientation. The crystallization rate and grain size of silicon films decrease, the uniformity of the surface increases with the increase of the epitaxial silicon film thickness.4. The influence of different annealed temperatures on the aluminum-induced crystallization of amorphous silicon films was studied. The silicon films with the structure of glass/α-Si/Al were prepared and annealed at 300 to 500 ℃ for 25 min in N2. The results show that the films were in the form of crystallized silicon after annealing. The grain size and crystallization rate of crystalline silicon films were gradually increased, surface uniformity was decreased as the annealing temperature gradually increased from 300 to 400 ℃. As the annealing temperature gradually increased from 400 to 500 ℃, all the grain size,crystallization rate and surface uniformity of crystalline silicon films were increased. The relationships between the grain size, crystallization rate and the band gap were obtained and shown in a three-dimensional surface plot. The surfaces of the crystallized silicon films with different microstructures show different colors, including black gray, brown, blue, red and yellow. The colorful solar cells can be prepared when the multichromatic films are used for the P layers of silicon solar cells.5. By applying the mechanism of Al-induced crystallization, the silicon films in the structure of glass/α-Si/Al were prepared by Al/Si co-sputtering. The Al and Si contents in films are controlled by the sputtering powers of Al and Si. The crystallized silicon films were prepared by annealing the sputtered films at 350 ℃ for 10 min in N2. The results show that the nc-Si silicon films were prepared at the Al/Si sputtering power ratio of 0.10, exhibiting the Raman scattering peak at 510 cm-1. The polycrystalline silicon films were obtained from annealing the a-Si/Al composite films prepared by magnetron co-sputtering at 350 ℃ for 10 min. The grain size and crystallization rate of the crystallized silicon films increased as the Al content increased, while the optical band gap was reduced. The crystallized silicon films with a crystallization rate from 50.81% to 88.58% and Si(111)crystal orientation was obtained by using co-sputtering followed by annealing at 350 ℃ for 10 min.