Overview of Crystallization of Amorphous Silicon Thin Films

The crystallinity of pc-Si thin film has great influence on the performance of thin film pc-Si solar cells. For a pc-Si thin film, the grain boundaries cause a lot of defects, which is so called strained bonds and dangling bonds. These defects act as trap states within the band gap and will degrade the performances of pc-Si solar cells, such as open circuit voltage, fill factor, and efficiency. Furthermore, the defects in pc-Si solar cells could result in many reliability issues. It is generally believed that enlarging the grain size and reducing the defect density are the most important key technologies to obtain high quality pc-Si thin films. Enlarging the grain size and reducing the defect density in pc-Si could make it approach the quality of single-crystalline silicon, which could result in a better performance of pc-Si devices. As a result, it is important to control the grain size and grain boundaries to fabricate high quality pc-Si thin films.

Recently, various methods, such as solid phase crystallization (SPC), metal induced crystallization (MIC), and laser crystallization, have been proposed for a-Si crystallization on glass material, which could be concluded that the a-Si thin films are re-crystallized into pc-Si thin film by additional energy. These kinds of low

temperature crystallization methods will be roughly reviewed in the following three sections.

1.2.1 Solid Phase Crystallization of Amorphous Silicon Thin Films

Silicon thin films deposited in the amorphous structure and then crystallized into the polycrystalline structure have been shown to have higher carrier mobility since the larger grain size compared to the direct-deposited pc-Si thin films [5]. Solid phase crystallization is a simple method to convert a-Si thin films into pc-Si thin films with large grains via furnace annealing by thermal energy for 24 hours at temperature of 600°C. With different annealing temperature and annealing time, the crystallinity of derived pc-Si thin film could be different.

Generally, the solid phase crystallization involves two distinct processes, the nucleation of seeds and grain growth into final polycrystalline silicon films [6]. The transformation proceeds within the amorphous matrix after an apparent incubation period by the nucleation and dendritic-like growth of crystal domain, and the final grain size could be larger if the nucleation rate is low and the grain growth rate is high [7].

Furthermore, the surface morphology is much smoother in the solid phase crystallization polycrystalline silicon thin films than in the as-doped ones. Despite the long crystallization durations of several tens of hours, large defect density still exists in the crystallized polycrystalline silicon thin films due to the low temperature process.

1.2.2 Metal Induced Crystallization of Amorphous Silicon Thin Films

Metal induced crystallization (MIC) is a method to lower the process temperature (<500°C) and shorten the process duration (<5hrs) of solid phase crystallization by introducing certain metal impurities [8]-[10]. The reaction between the metal and amorphous silicon occurs at an interlayer by diffusion and lowers the crystallization temperature. The enhancement of crystallization is due to an interaction of the free electrons from the metal with covalent Si bonds near the growing interface.

Recently, several metals have been proposed to realize the MIC process, such as aluminum (Al), aurum (Au), platinum (Pt), and nickel (Ni), etc. Among various metals, Al has been chosen for creating pc-Si thin films for thin film pc-Si solar cells due to its promising very large grains (1 – 100μm) and the low processing temperature [11]. This process is also called aluminum induced crystallization (AIC). When Al is deposited on a-Si, electronic screening weakens the covalent bonds of the a-Si, facilitating the diffusion of Si atoms into the metal film during heating, where the Si atoms will find some preferred nucleation sites at the metal grain boundaries. Because of the continuous supply of Si atoms, the Si grains will continue to grow at these sites until they contact each other and form a continuous film which ultimately results in layer inversion. For Al, this process can start immediately because of the solubility of Si in the metal film while the solubility for Ag is negligible even at elevated temperatures of 1350°C. More detail, considering the temperature of metal-Si eutectic, a-Si thin film can be crystallized below 500°C.

1.2.3 Laser Crystallization of Amorphous Silicon Thin Films

Laser crystallization has been receiving considerable attention in fabrication of silicon-on-insulator devices for microelectronics and thin-film transistors for displays [12]. Laser crystallization can produce pc-Si thin film low density of intra-grain defects.

The laser crystallization is a liquid phase crystallization process which is contrast to above discussed two solid phase crystallization process such as SPC and MIC.

Therefore, many studies in laser crystallization of a-Si films have been using various kinds of laser techniques, such as CO2, Ar, Nd:YAG, Nd:YVO4, excimer, and femtosecond lasers[13][14].Among these laser techniques, excimer laser annealing is the widely used to prepare pc-Si thin films thanks to its high pulsed-laser power and the large absorption coefficient for a-Si in the UV light region. According to the gas mixture used in the laser tube, excimer laser radiation of output wavelengths between 157 - 351 nm were obtained by the transient high voltage discharge with short pulse duration.

Another unique advantage of lasers is the strong optical absorption of UV light in silicon. As a result, most of the incident laser energy is absorbed close to the surface of the thin film without causing severe thermal strain on the substrate. The strong optical absorption of the UV light in silicon and short pulse duration of the excimer laser imply that high temperature can be produced in the silicon surface region, causing rapid melting and solidification, without significant heating of the substrate. This avoids diffusion of impurities from the substrate into the silicon thin film. This technology yield high quality and large-grained pc-Si thin film for high-performance pc-Si TFTs on glass or plastic substrate with high throughput. Owing to the advantageous features of excimer laser crystallization for large area microelectronics fabrication, many studies

have been done to understand the dynamic kinetics and transformation mechanisms of the laser crystallization of a-Si thin films. The characteristics of pc-Si thin film are related to the process conditions of laser crystallization, such as laser energy density, pulse duration, number of shots per unit area, crystallization ambient, and substrate temperature. Moreover, the initial conditions of a-Si precursor film has a profound effect on the properties of the resulting pc-Si film, including thickness, hydrogen and impurity content of a-Si film [12]-[15].