Study on the mechanism of rapid solid-phase recrystallization of hydrogenated amorphous silicon film by rapid thermal processing
Introduction
Hydrogenated microcrystalline silicon film (μc-Si:H) is an important candidate material in a wide range of electronic and opto-electronic applications, such as thin film transistors for flat panel displays [1], [2], [3] and as an intrinsic layer for silicon thin film solar cells [4], [5]. The use of μc-Si:H films instead of a-Si:H films as the active layer in thin film transistors and silicon thin film solar cells has attracted much attention due to their high carrier mobility and small optically induced degradation under illumination [6], [7], [8], [9]. μc-Si:H films are commonly deposited by radio-frequency PECVD (RF-PECVD) [10], [11] and very high frequency PECVD (VHF-PECVD) techniques [12], [13] through carefully controlled deposition. However, these two techniques, due to their complexity and high cost, are relatively difficult to apply to large-scale productions of μc-Si:H film solar cells. Even though the as-deposited silicon film on glass substrates at low temperature by PECVD and VHF-PECVD is usually amorphous instead of microcrystalline without a careful control over deposition parameters, solid-phase recrystallization technology can easily resolve this problem. Among the solid-phase recrystallization technologies available, traditional furnace annealing is widely applied to solid-phase recrystallization [14]. However, the technique usually takes several hours, even at high temperatures, before the final product is realized. Rapid thermal processing (RTP), widely applied in the microelectronic industry since the 1990s, has recently been applied to silicon thin film solar cells. The process significantly shortens the solid-phase recrystallization time, taking several minutes at most to produce hydrogenated amorphous silicon (a-Si:H) films [15], [16]. So far, general agreement on the mechanism of rapid solid-phase recrystallization (RSPR) of a-Si:H films by RTP has not been reached. It is generally argued that aside from thermal effects, quantum-optical effects also play an important role in the process [17].
To foster deeper understanding of the RSPR mechanism of a-Si:H films by RTP, high-quality a-Si:H films were deposited on quartz glass substrates by RF-PECVD techniques. The films were then subjected to thermal RTP at 800 °C for 3 min. As confirmed by X-ray diffractometry (XRD) and Raman spectrometry, μc-Si:H films were obtained after the annealing procedure. The RSPR mechanism of a-Si:H films by RTP was carefully studied by analyzing the excited quantum-optical process of the film.
Section snippets
Experiments
Films made of a-Si:H were deposited on quartz glass substrates at room temperature in a traditional RF-PECVD room. Before film deposition, the vacuum chamber was evacuated to a base pressure below 2 × 10−4 Pa to decrease the possibility of oxygen contamination. During film deposition, a flow rate ratio of H2 to SiH4 of 97:3 and a working pressure of 133 Pa were maintained. The deposited films were then subjected to thermal RTP at 800 °C for 3 min. The film crystalline structure was characterized by
Results and discussion
The Raman pattern of the as-deposited silicon film is shown in Fig. 2. A 10 mW He–Ne laser source was used as Raman spectrometer signal source to eliminate undesirable effects. As seen in Fig. 2, a distinct amorphous wave envelope centered at 480 cm−1 was observed, indicating that the deposited silicon film was amorphous. Fig. 3 shows the optical reflectivity of the deposited a-Si:H film. Clearly, a strong oscillation occurred in the film transparent region. A film thickness of about 600 nm was
Conclusions
High-quality μc-Si:H films on glass substrates were obtained by the combination of RF-PECVD and RTP techniques as confirmed by Raman spectrometry and X-ray diffractometry. RTP technique was effective to transform a-Si:H into μc-Si:H at 800 °C for 3 min. Theoretically analyzing the film quantum-optical process, the RSPR mechanism of a-Si:H film by RTP was theoretically mainly attributed to the interaction between short-wavelength photons and ground-state silicon, SiH2 and SiH3 radicals. The
Acknowledgements
The work is supported by the National Natural Science Foundation of China (No. 60807001) and Basic Research Program of China (973) (No. 2006CB202601).
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