脉冲功率对含硅量子点SiCx薄膜物相结构及光谱特性的影响

基于硅量子点（Si-QDs）的全硅叠层太阳电池被认为是最有潜质的高效太阳电池之一。目前所报道的硅量子点薄膜存在硅量子点数密度低、缺陷多等问题，限制了硅量子点太阳电池的光电转换效率。微波退火(microwave annealing, MWA)被认为是一种有益于制备纳米结构材料的方法。微波退火的非热效应可以降低形核能，改善薄膜的微结构和光电性能。因此，采用磁控共溅射技术并结合微波退火工艺，在不同的脉冲功率下制备了含硅量子点SiC_x薄膜；采用掠入射X射线衍射(GIXRD)、拉曼(Raman)光谱、紫外-可见-近红外分光光度计和光致发光(PL)光谱表征薄膜的物相结构及光谱特性；研究不同脉冲功率对硅量子点数密度和性能的影响，进而改进磁控共溅射工艺，制备硅量子点数密度较高和性能良好的薄膜。样品的GIXRD谱和Raman谱均显示其中存在硅量子点，其强度先增大后减小；通过谢乐(Scherrer)公式估算出硅量子点尺寸呈现先增大后减小的规律，脉冲功率为80 W时尺寸达到最大(8.0 nm）。在Raman光谱中还观察到中心位于511 cm-1处出现硅量子点Si-Si横向光学振动模式的拉曼峰，其强度也呈现先增大后减小的趋势；对拉曼光谱做最佳高斯(Gauss)分峰拟合，得出薄膜的晶化率均高于62.58%，脉冲功率为80 W时制备的薄膜具有最高的晶化率(79.29%)。上述分析表明薄膜中均有硅量子点的形成，且数量先增加后减小，脉冲功率为80 W时硅量子点数量最多。通过测量样品的透射率T、反射率R等光学参数，利用Tauc公式估算出薄膜的光学带隙，发现带隙值随溅射功率的增加先减小后增大，在脉冲功率为80 W时最小(1.72 eV)。硅量子点尺寸与光学带隙成反比，说明薄膜中的硅量子点具有良好的量子尺寸效应。通过PL光谱分析样品的发光特性，对其做最佳高斯拟合，发现样品中均有6个发光峰。结合Raman光谱的分析结果，可以得出波长位于463~624 nm的发光峰源于硅量子点的作用；而波长位于408和430 nm的发光峰则源于薄膜内部的缺陷态，峰位没有偏移，但强度有变化。根据发光峰对应的波长可计算其能带分布，从而确定缺陷态类型：408 nm的发光峰归因于≡Si°→E_v电子辐射跃迁，430 nm的发光峰则归因于≡Si°→≡Si-Si≡的缺陷态发光。还研究了硅量子点的尺寸对发光峰移动的影响。结果表明，随硅量子点尺寸变小（大），发光峰蓝移（红移）。综上，溅射功率为80 W时制备的含硅量子点SiC_x薄膜性能最佳。研究结果为硅量子点太阳电池的后续研究奠定了基础。

Effect of Pulse Power on the Phase Structure and Spectral Properties of SiCx Thin Films Containing Si Quantum Dots

All-silicon tandem solar cells based on silicon quantum dots (Si-QDs) are considered to be one of the most promising high efficiency solar cells. In recent years, Si-QDs films with low Si-QDs density and many defects were reported. Thence, the photoelectric conversion efficiency of Si-QDs solar cells waslimited. Microwave Annealing (MWA) is considered to be a useful method to prepare nanostructured materials. The non-thermal effect of MWA can reduce energy for nucleation and improve the microstructure and photoelectric properties of the films. In this paper, SiC_x thin films containing Si quantum dots were prepared via magnetron co-sputtering technique and MWA with different pulse power.The phase structure and spectral properties of Si-QDs films were characterized by grazing incidence X-ray diffraction (GIXRD), Raman, photoluminescence (PL) and spectrophotometer. The influence of different pulse power on the Si-QDs density and performance was studied systematically.Thin films with highdensity and good performance weredeposited by improving the magnetron sputtering process.The GIXRD and Raman spectra all showed that the Si-QDs existed in the samples, and their intensities first increased and then decreased. By Scherrer’s formula, it was estimated that the size of Si-QDs increased initially and then decreased, and the maximum size of Si-QDs (7.98 nm) wasobtained when the sputtering power was 80 W. The centers of Raman peaks areat 511 cm-1. This is ascribed to Si-Si lateral optical vibration modeand its intensity is also increased first and then decreased. The optimum Gauss peak fitting was used for the Raman spectra. It showed that the crystalline fraction was higher than 62.58%, and the highest crystallinefraction (79.29%) was gained when the power was 80 W. The above analysis showed that Si-QDs formed in the films and the size of Si-QDs first increases and then decreases. The maximum number of Si-QDs was acquiredwith the power of 80 W. The optical bandgap was estimated by Tauc formula. These bandgaps were going to decrease and then increase with the increase of power. The bandgap reachedminimum value (17.2 eV) with the power of 80 W. The Si-QDs size was inversely proportional to the band gap, indicating that the Si-QDs in the films had good quantum confinement effect. The luminescence properties of the samples were analyzed by the PL spectra, the optimum Gauss peak fitting was used. It was found that there were 6 luminescence peaks. Combine with the results of Raman spectrum, the luminescence peaks between 463~624 nm were derived from the role of the Si-QDs. The luminescence peaks between 408 and 430 nm originated from the defect state inside the films withoutthe shift of peak position, while the intensity varies. The distribution of the energy band gap were calculated according to the wavelength of the luminescence peak. Thus, the types of the defect state were determined, the luminescence peak at 408 nm is attributed to the electron radiation transition of ≡Si°→E_v, and the luminescence peak at 430 nm is attributed to ≡Si°→≡Si-Si≡ defect state luminescence. The Si-QDs size on the luminescence peak shift was also studied. The results show that blueshift (redshift) of luminescence peak occurred with the size of Si-QDs becoming smaller (larger). In conclusion, SiC_x films with Si-QDs prepared at the sputtering power of 80 W exhibited the best performance. The research results laid the foundation for the follow-up study of Si-QDs solar cells.