Polycrystalline silicon thin film made by metal-induced crystallization
Introduction
Polycrystalline silicon thin films are attracting more interest in electronic devices such as thin film transistors, solar cells, and sensors in the recent years. Many groups have reported on the various low-temperature poly-Si growth techniques such as excimer laser annealing (ELA) [1], solid-phase crystallization (SPC) [2] and rapid thermal annealing (RTA) [3]. Recrystallization can be distinguished from the process temperature. High-temperature poly-Si is based on a single crystalline large-scale integration (LSI) process, and the process temperatures are as high as 900 °C. Therefore, high-temperature process requires the high-power consumption and equipment of quartz furnace and forces to choose the substrate such as silicon and quartz. On the other hand, low-cost glass substrates could be used for low-temperature poly-Si, since the maximum fabrication temperature is below 600 °C. In order to crystallize the amorphous silicon film, annealing treatments are conventionally carried out at temperatures in the range 850–1000 °C for a fixed time. However, high crystallization temperature induces the substrates like glass to soften or to get bent. To overcome this problem, metal-induced crystallization has been investigated for a long time. Metals like aluminum, nickel, gold, and silver are very much used to decrease the crystallization temperature below 600 °C. Some of the metals that have been investigated for eutectic-phase-assisted crystallization are Au, Ag, Al, In, Ga, Zn, and Sn [4], [5], [6], [7], [8], [9], [10]. The well known driving force behind eutectic reaction metal-induced crystallization is the difference of the free energy between amorphous and crystalline silicon material during the reduction of the free energy of silicon [11]. aluminum-induced crystallization (AIC) of amorphous silicon has been suggested as an alternative crystallization process for the formation of continuous polycrystalline films on glass. In the case of aluminum, the crystallization temperatures as low as 150 °C have been reported [12]. However, it has an important problem, which is not able to induce the formation of extended continuous polycrystalline silicon films for which the time duration should be changed. In the case of solar cells, with the crystallization of amorphous silicon at low temperature along with low-cost substrates, it is possible to reduce the cost of the solar cells. In this work, we have investigated AIC of amorphous silicon below the eutectic temperature of 577 °C as a function of temperature on glass substrate.
Section snippets
Experiments
In the present work, we prepared the structure of a-Si/metal/glass, where the a-Si was crystallized below the eutectic temperature of 577 °C through a rapid thermal process. The substrates used for this structure were made by Corning 1737 glass which has its strain point at 666 °C. The substrates were cleaned by a standard process. The initial layer of Al was deposited at room temperature by thermal evaporation method. Before deposition, the chamber was pumped in to a background pressure of 2×10−5
Results and discussion
Fig. 1 shows the Raman spectroscopy after RTA crystallization as a function of time duration at 550 °C. With increase in crystallization time, the peak around 521 cm−1 becomes sharp and high. When the crystallization time is short, we observed that the structure of film is in an amorphous phase for the most part from the shoulder around 480 cm−1 as shown the Fig. 1(a), (b). We calculated Xc using the peak at 480 cm−1, indicating amorphous silicon and the peak at 521 cm−1, indicating single crystal
Conclusion
In this article, we investigated the aluminum-induced crystallization of amorphous Si. The as-deposited silicon on the Al/glass substrate was fully crystallized below the eutectic temperature which promotes the grain growth. This crystallization is induced by the diffusion of Al into Si and changes Si–Si bonding from covalent to metallic and contributes to a large reduction in the activation energy, which also helps to make the bonding states in a-Si to become like those in crystalline Si
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