Analysis of the ellipsometric spectra of amorphous carbon thin films for evaluation of the sp3-bonded carbon content
Analysis of the ellipsometric spectra of amorphous carbon thin films for evaluation of the sp3-bonded carbon content
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DOI：
10.1016/s0925-9635(97)00341-5
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发表时间：
1998-07-01
影响因子：
4.1
通讯作者：
Robertson, J
中科院分区：
文献类型：
--
作者：
Lee, J;Collins, RW;Robertson, J
用椭圆偏振光谱法（SE）测量了一系列厚度为100-300埃的非晶碳（a-C）薄膜的光学性质和光学带隙。这种膜表现出20至76原子%的宽范围的sp（3）键合碳含量，如通过电子能量损失谱（EELS）测量的。a-C薄膜的Tauc光学带隙从0.65eV单调增加到20 at.% sp（3）C至2.25 eV，76 at.% sp（3）C.使用不同的有效介质理论（EMT）分析了椭圆偏振角（1.5-5 eV）中的光谱，应用a-C介电函数的简化光学模型，假设复合材料具有sp（2）C和sp（3）C成分。最广泛使用的EMT，即Bruggeman的EMT（三维各向同性筛选），产生与EELS获得的那些单调相关的sp（3）C的原子分数。然而，SE分析的结果范围为10至25原子%。高于那些从ELS。事实上，我们已经发现使用Bruggeman EMT从SE得到的sp（3）C的体积百分比与从EELS得到的sp（3）C的原子百分比显示出良好的数值一致性。通过使用光学模型，其中a-C的介电函数被确定为sp（2）C和sp（3）C组分的介电函数的体积分数加权平均值，减少了SE-EELS差异。(C)1998年Elsevier Science S.A.
Using spectroscopic ellipsometry (SE), we have measured the optical properties and optical gaps of a series of amorphous carbon (a-C) films similar to 100-300 Angstrom thick, prepared using a filtered beam of C+ ions from a cathodic are. Such films exhibit a wide range of sp(3)-bonded carbon contents from 20 to 76 at.%, as measured by electron energy loss spectroscopy (EELS). The Tauc optical gaps of the a-C films increase monotonically from 0.65 eV for 20 at.% sp(3) C to 2.25 eV for 76 at.% sp(3) C. Spectra in the ellipsometric angles (1.5-5 eV) have been analyzed using different effective medium theories (EMTs) applying a simplified optical model for the dielectric function of a-C, assuming a composite material with sp(2) C and sp(3) C components. The most widely used EMT, namely that of Bruggeman (with three-dimensionally isotropic screening), yields atomic fractions of sp(3) C that correlate monotonically with those obtained from EELS. The results of the SE analysis, however, range from 10 to 25 at.% higher than those from EELS. In fact, we have found that the volume percent sp(3) C from SE using the Bruggeman EMT shows good numerical agreement with the atomic percent sp(3) C from EELS. The SE-EELS discrepancy has been reduced by using an optical model in which the dielectric function of the a-C is determined as a volume-fraction-weighted average of the dielectric functions of the sp(2) C and sp(3) C components. (C) 1998 Elsevier Science S.A.