过渡区p型氢化硅氧薄膜结构和光电特性的研究

采用射频等离子体增强化学气相沉积技术,利用二氧化碳(CO_2)、氢气(H_2)、硅烷(SiH_4)和乙硼烷(B_2H_6)作为气源,制备出一系列p型氢化硅氧薄膜.利用拉曼光谱、傅里叶变换红外光谱和暗电导测试,研究了不同二氧化碳流量对薄膜材料结构和光电特性的影响,获得了从纳米晶相向非晶相转变的过渡区P层.研究表明:随着二氧化碳流量从0增加到1.2 cm~3·min~(-1),拉曼光谱的峰值位置从520 cm~(-1)逐渐移至480 cm~(-1).材料红外光谱表明,随着二氧化碳流量的增加,薄膜中的氧含量逐渐增加,氢键配置逐渐由硅单氢键转换为硅双氢键.P层SiO:H薄膜电导率从3S/cm降为8.3×10~(-6)S/cm.所有p型SiO:H薄膜的光学带隙(Eopt)都在1.82—2.13 eV之间变化.在不加背反射电极的条件下,利用从纳米晶相向非晶相转变的过渡区P层作为电池的窗口层,且在P层和I层之间插入一定厚度的缓冲层,制备出效率为8.27%的非晶硅薄膜电池.

Structural and optoelectronic properties of p-type SiO:H films deposited in transition zone

P-type hydrogenated silicon oxide (p-SiO_x:H) films are prepared by radio frequency plasma enhanced chemical deposition with various CO₂ flow rates. We use gas mixtures of carbon dioxide (CO₂), hydrogen (H₂), silane (SiH₄) and diborane (B₂H₆) as reaction source gases. For all experiments the substrate temperature, pressure and power density are fixed at 200 ^oC, 200 Pa and 200 mW/cm², respectively. The films are deposited on Corning Eagle 2000 glass substrates for optoelectronic measurements and on crystalline Si wafers for Fourier transform infrared (FTIR) measurement. The structural, optical and electronic properties of the films are systematically studied as a function of CO₂ flow rate. The CO₂ flow rate is varied from 0 to 1.2 cm³· min^-1, with all other parameters kept constant. It is shown that with the CO₂ flow rate increasing from 0 to 1.2 cm³· min^-1, the Raman peak shifts from 520 cm^-1 to 480 cm^-1 and corresponding crystalline volume fraction decreases from 70% to 0. In addition, the FTIR spectrum shows that the oxygen content increases from 0 to 17% and the hydrogen bond configuration gradually shifts from mono-hydrogen (Si-H) to di-hydrogen (Si-H₂) and (Si-H₂)_n complexes in the film. What is more, with the incorporation of oxygen, the optical band gap of each of all p-type SiO:H films increases from 1.8 eV to 2.13 eV, while the dark conductivity decreases from 3 S/cm (nc-Si:H phase) to 8.310^-6 S/cm (a-SiO_x:H phase). Furthermore, the oxygen incorporation tends to disrupt the growth of silicon nanocrystals due to the created dangling bonds that arises from an increased structural disorder. This leads to microstructural evolution of SiO:H film from a single nanocrystalline phase into first a mixed amorphous-nanocrystalline and subsequently into an amorphous phase. At a certain threshold of CO₂ flow rate, a transition from nanocrystalline to amorphous growth takes place. The transition from nanocrystalline to amorphous silicon is confirmed by Raman and FTIR spectra. In the transition region or crystalline volume fraction of about 45%, Raman spectrum also reveals that the a mixture of nanocrystalline silicon and amorphous silicon oxide (a-SiO_x:H) phase exists in the film. This means that nanocrystalline silicon oxide (nc-SiO:H) is a two-phase structural material consisting of a dispersion of silicon nanocrystals (nc-Si) embedded in the amorphous SiO_x network. As is well known, the oxygen-rich amorphous phase can help enhance the optical band gap, while the nc-Si phase contributes to high conductivity. Finally, it is the SiO:H film deposited at phase transition that can realize a relatively high dark conductivity (about S/cm) with a wide optical band gap of 2.01 eV in the film. By using the transition p-layer as the window layer in conjunction with a suitable buffer thickness, we obtain a thin film solar cell with an open-circuit voltage of 890 mV, a short-circuit current density of 12.77 mA·cm^-2, fill factor of 0.73, and efficiency of 8.27% without using any back reflector.