Observation of Blue Emission from ECR-CVD Deposited Amorphous Hydrogenated Silicon Carbide
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Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society
measurements were carried out at room temperature using an Ar+ ion laser source at near normal incidence to the sample. Several excitation energies Eex=2.41, 2.49, 2.54, 2.60, 2.71 and 3.41eV were used, and the PL spectra were detected in the reflection direction using a water-cooled Hamamastu R2949 photomultiplier tube (PMT) based on single-photon counting technique through a 1/4m double monochromator (Digikrom DK242). The PL spectra were corrected taking into account the effect of the combined response of the PMT and monochromator. RESULTS Figure 1 shows the E 04 gap and Tauc gap Eg as a function of the microwave power. The optical gaps decrease with increasing microwave power up to 600W, beyond which both increase at higher microwave power. The optical gaps of these a-Sil_.C,:H films have been shown to critically depend on the fraction of Si and C incorporated [6,7], besides other factors such as the H fraction and the predominating bonding configuration of the carbon atoms [8]. Information on the exact fraction of each element in these films is not available at present. However, comparing the E 04 and the Tauc gaps for our films with those reported in the literature [6,7], it can be deduced that our films are carbon-rich, with carbon fraction x estimated in the range of 0.7 and above. From the IR absorption and Raman scattering results [9], it was found that the Si fraction incorporated in the films increased with the microwave power. Thus, films deposited at lower microwave power are expected to contain a larger carbon fraction, mainly in polymeric form as suggested by the IR absorption results, and can account for the large optical gap observed. It is noted that wide gap polymeric a-C:H films with E 04 as high as 4.3eV have been previously reported by Rusli et al. [10]. With increasing microwave power, the decrease in the opticl gap can be attributed to an increase in the carbon sp2 component in the films, as deduced from the ratio of the D peak to G peak strength of the Raman spectra [9]. At higher microwave power, the Si fraction increases and this helps promote structural ordering into a more sp3 network, and can account for the increased optical gap observed [8]. The larger optical gap can also be due to the presence of abundant hydrogen atoms, particularly at higher microwave power, that effectively etch off weaker sp 2 bonded carbon. 3.8
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Mcroave Poer (W) Fig. 1. The E 04 gap and Tauc gap Eg as a function of microwave power. The Urbach tail width E0 is shown in Fig. 2 where it can be seen that E5 exhibits a trend with the microwave power that is opposite to that seen for the optical gap. Therefore, a monotonic relation between E0 and the E 04 gap (or Tauc gap) exists, with E0 decreases at increasing optical 530
gap. The Urbach tail width E0 is a measure of disorder in the materials and is expected to increase for a-Sil.Cx:H films containing a mix
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