Optical Characterization of As 2 Te 3 Films for for Optical Interconnects
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surface catalyzed reaction between As(N(CH 3 )2 )3 and ((CH 3 )3 Si) 2 Te which results in the formation of stoichiometric As 2 Te3 films on the surface; the other reaction by-products are volatile and do not incorporate [4]. The reaction readily takes place at room temperature and the resulting films are amorphous as evidenced by the lack of any peaks in 20 x-ray diffraction spectra. Deposition at higher temperatures result in films that are at least partly crystalline in nature since the x-ray spectra exhibited broad peaks. Sample characteristics are listed in Table 1. The substrates used were p-type Si with resistivity between 50-200 2-1 cm- 1 and orientation [100] tilted 4 degrees towards [110], and single crystal KBr. Table 1. Characteristics of samples. Sample Substrate
Growth Temperature (K)
Film thickness (gim)
Eg (eV)
B (cm-leV-1)
7-1 9-1
KBr KBr
293 373
1.1 0.54
0.9 0.83
4.9x10 5 4.0x10 5
13-1
Si
293
0.257
0.94
7.9x10 5
Comments
amorphous partially crystalline amorphous
For the sample 13-1, the thickness was determined from step profiler measurements calibrated with NIST traceable standards. The thicknesses of samples on KBr were determine from the interference fringes and were within experimental error of step profiler results. In addition, the variation of film thickness across the samples were estimated to be about t 5%. RESULTS Figures 1 and 2 show data on transmission (T) and reflection (R). Interference fringes are observed in the high-transmission region of the spectra for both Fig. 1 and 2 although for different reasons. The spectra from sample 7-1 in Fig. 1 shows the interference effect in the As 2 Te3 film. In Fig. 2, sample 13-1, reveals a clear interference pattern related not to As2Te 3 but to the Si substrate. Because of the high degree of wafer thickness uniformity, the fringes are well resolved despite the fact that the substrate thickness is much greater (-75 jim) than the thickness of the film (- 0.3 jim). The wavelength dependence of the refractive index for Si has been determined from the condition for maximum and minimum transmission: nd =m Xxt / 4, (1) where n is the refractive index at the wavelength of extrema Xext, and d is the wafer thickness. The order of the fringes, m, can be determined from the wavelengths of two adjacent transmission or reflection extrema in the spectral range where there is no dispersion of n. The obtained results shown in Figure 3 correlate quite well with optical characteristics of silicon membranes presented at this meeting [5] where this procedure is also described. It is obvious that the film's absorption coefficient cannot be accurately extracted from the transmission spectra in the spectral range where fringes become visible (at wavelengths greater than 0.7-0.9 jm for our samples). Instead, the interference technique has been utilized in this case. The technique has been described previously for a thin film deposited on a partially absorbing substrate [6,7]. It is based on new self-consistent data-analysis algorithms for simultaneou
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