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TEM analysis of multilayered nanostructures formed in the rapid thermal annealed silicon rich silicon oxide film

机译：快速热退火富硅氧化硅膜中形成的多层纳米结构的TEM分析

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Silicon (Si) nanoparticles (NPs) embedded in an ultrathin silicon rich silicon oxide (SRSO) film through the thermal annealing process has emerged as a highly absorbing layer for third-generation solar cells 1. The concept of using Si NPs is to achieve a band gap tunable absorber layer by controlling the size and structure of Si NPs because of the quantum confinement effect 2. In our study, a multilayer stack of silicon oxide with 35 periods of alternating layers of 1-nm thick near-stoichiometric and 3-nm thick Si-rich hydrogenated silicon oxide were deposited on fused quartz substrate by plasma-enhanced chemical vapor deposition (PECVD) method. Two samples were annealed using a rapid thermal annealing (RTA) furnace in forming gas atmosphere (90% N2 + 10% H2) for 210s and 270s respectively. From the Raman spectroscopy, a reduction in crystallinity of Si has been discovered from 210s annealed sample to 270s annealed sample (shown in Figure 2). The goal of transmission electron microscopy (TEM) analysis is to investigate the nanostructural change of Si in these two annealed samples and try to correlate the TEM observations to the Raman spectroscopy results. As the dimension of the Si nanostructures formed in SRSO films is in nanometer-scale, the energy-filtered TEM (EFTEM) tomography technique using the low-loss signals in electron energy-loss spectroscopy (EELS) has been applied as a powerful technique to correlate the precipitated Si nanostructures to the phase transformation mechanisms in the thermally annealed SRSO films 3. In this case, EFTEM spectrum-imaging (SI) technique was applied to characterize the Si nanostructures formed in SRSO films by different annealing times. The EFTEM SI dataset was acquired from -4eV to 40eV using a 2eV energy slit and the reconstructed zero loss peak (ZLP) was used to calibrate the spectra shift. Si plasmon images were extracted by fitting a Gaussian into the low-loss region with a peak position at 16.7 eV 4 and FWHM of 4.5 eV. In order to analyze the multilayer structures at different annealing durations, the TEM samples were prepared in cross sectional geometry using the conventional polishing and ion milling methods. Figure 1 shows the EFTEM images extracted from the Si plasmon peak, in these images Si appears as bright contrasts. For shorter annealing time, an alternating bright and dark contrast can be observed which indicates that the multilayer structure still remains whereas for longer annealing time, Si shows nanoparticles like contrast. The continuous layer like contrasts shown in Figure 1(a) indicates the overlapping of the contrasts generated by small Si crystallites in a very high density. After longer annealing time (Figure 1(b)), the small Si crystallites grow in size but may take overall less volume fraction due to the Ostwald ripening process. Therefore, it explains the reduction in crystallinity of Si discovered from 210s annealed sample to 270s annealed sample by Raman. However, such a reduction in Si crystallinity was not observed in nitrogen annealed SRSO films, this indicates that samples annealed in the forming gas environment follow a different crystallization mechanism and hydrogen must play a decisive role during the Si crystallization at the initial stage.
机译：通过热退火工艺嵌入超薄富硅氧化硅（SRSO）膜中的硅（Si）纳米颗粒（NPs）已经成为第三代太阳能电池1的高吸收层。使用Si NPs的概念是实现通过限制量子点效应来控制Si NP的尺寸和结构来实现带隙可调吸收层2.在我们的研究中，氧化硅的多层堆叠具有35个周期的交替层，每个交替层的厚度为1 nm，接近化学计量比，厚度为3 nm。通过等离子体增强化学气相沉积（PECVD）方法在熔融石英衬底上沉积厚的富Si氢化硅氧化物。使用快速热退火（RTA）炉在形成气体气氛（90％N2 + 10％H2）中分别退火两个样品210s和270s。从拉曼光谱法中，发现硅的结晶度从210s退火样品降低到270s退火样品（如图2所示）。透射电子显微镜（TEM）分析的目的是研究这两个退火样品中Si的纳米结构变化，并尝试将TEM观察结果与拉曼光谱结果相关联。由于SRSO膜中形成的Si纳米结构的尺寸为纳米级，因此利用电子能量损失谱（EELS）中的低损失信号的能量过滤TEM（EFTEM）层析成像技术已被广泛应用于将沉淀的Si纳米结构与热退火SRSO膜3中的相变机理相关联。在这种情况下，采用EFTEM光谱成像（SI）技术来表征通过不同退火时间在SRSO膜中形成的Si纳米结构。使用2eV能量狭缝从-4eV至40eV采集EFTEM SI数据集，并使用重建的零损耗峰（ZLP）校准光谱偏移。通过将高斯拟合到峰值位置为16.7 eV 4且FWHM为4.5 eV的低损耗区域来提取Si等离子体激元图像。为了分析不同退火时间下的多层结构，使用常规抛光和离子铣削方法以横截面几何形状制备了TEM样品。图1显示了从Si等离子体激元峰提取的EFTEM图像，在这些图像中，Si表现为明亮的对比度。对于较短的退火时间，可以观察到交替的明暗对比，这表明仍保留多层结构，而对于较长的退火时间，Si显示出类似对比度的纳米颗粒。如图1（a）所示的类似对比的连续层表示由很小的硅微晶以非常高的密度产生的对比的重叠。经过较长的退火时间（图1（b））后，小的Si晶粒尺寸增大，但由于奥斯特瓦尔德（Ostwald）熟化过程，其整体体积分数可能会减少。因此，这解释了由拉曼发现的从210s退火样品到270s退火样品发现的Si结晶度降低。然而，在氮气退火的SRSO膜中未观察到Si结晶度的降低，这表明在形成气体环境中退火的样品遵循不同的结晶机理，并且氢在初始阶段的Si结晶过程中必须起决定性作用。

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