Variation of Bandgap in Nanocrystalline Silicon as Monitored by Subgap Photoluminescence
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deposition excitation frequency is increased with a corresponding decrease in FWHM from 0.15 to 0.12 eV, respectively [2]. However, a lower energy shift in optical absorption was not observed, nor did they look for a correlation with the variation of any other physical property. Here, we report a broader range of energy variation for PL from our ECR-PECVD deposited nc-Si films accompanied with an energy shift in optical absorption and variation in dark conductivity and crystallite size. It has been argued that this PL peak in question could arise from the radiative transitions between bandtail states as in the case of -1.3 eV PL in a-Si:H or it may be associated with structural defects [2,3]. Our observations support both possibilities. EXPERIMENT The nc-Si films were grown on Coming 1737 glass substrates using a PlasmaTherm SLR 7700 ECR-PECVD system with the deposition conditions as listed in Table I. An X-ray diffractometer (Phillips X'pert) with a glancing incidence optics was employed to investigate the crystallinity of the films as well as the broadness of the (111) diffraction peak, indicative of the grain size. The PL signal was obtained by a Fourier Transform interferometer (BIO-RAD FTS40) and a LN 2 cooled Ge detector. The PL samples were cooled to 77-150 K in LN 2 and N 2 vapor and were excited by an Argon laser (488 nm). The absorption coefficient was derived from the transmission and reflectance data (obtained by a Perkin Elmer Lambda 9 spectrometer) using a geometric optical analysis. 500 A thick Al stripes of 12 mm length and 1 mm seperation were deposited by thermal evaporation on the films for dark and light conductivity measurements. The activation energy of the films was extracted from the temperature dependence of the dark conductivity from 25 oC to 175 oC. The photoconductivity measurements were performed under 291 Mat. Res. Soc. Symp. Proc. Vol. 507 © 1998 Materials Research Society
Table I. Deposition conditions of nc-Si films. 180, 260, 340 °C Microwave power 600 W RF power (substrate bias) 30 W Deposition chamber diameter 14" H 2 flow rate 40 sccm SiH 4 flow rate 2 sccm Total pressure 8 mTorr SjuU.uact t•cIp•perturi
simulated AMI .5 light (100 mW/cm 2 ) from an ELH quartz-halogen lamp. Microstructure was studied by TEM to deduce the grain size. RESULTS AND DISCUSSION In Fig. 1 the lower energy shift of the PL with increasing deposition temperature is shown for samples A, B, C, which are -4500 A thick nc-Si films deposited at 180, 260 and 340 oC, respectively. The redshift of the PL peak from 0.99 eV to 0.86 eV is accompanied with a decrease in FWHM of from 0.13 eV to 0.12 eV only. On the other hand a much more pronounced decrease in the bandwidth is introduced when the sample A is exposed to a furnace annealing of 72 hours at 600 °C; i.e., sample D with a FWHM of 0.09 eV and a peak at 0.81 eV. However, PL from sample D was observable only after hydrogenation in an ECR chamber at 300-350 °C for 30 min. Hence, putting H back into the film after annealing leads to a decrease in competing pat
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