Characterization of a-Si:H Near a-Sin x :H/a-Si:H Interface by Photoluminescence Spectra
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CHARACTERIZATION OF a-Si:H NEAR a-SiNx:H/a-Si:H INTERFACE BY PHOTOLUMINESCENCE SPECTRA TETSUZO YOSHIMURA, KOUICHI HIRANAKA, TADAHISA YAMAGUCHI, SHINTARO YANAGISAWA, AND KUNIHIKO ASAMA FUJITSU LABORATORIES LTD., 10-1 Morinosato-Wakamiya, Atsugi, Japan ABSTRACT
The influence of the a-SiNx:H composition on the near-interface properties of the a-SiNx:H/a-Si:H layered structure was studied by photoluminescence measurements and the results were compared with a-Si TFT characteristics. It was found that the cw photoluminescence spectra of a-Si:H near the interface tend to shift to the low-energy side with an increase in nitrogen content of the a-SiNx:H layer. Time-resolved photoluminescence measurement revealed that the low-energy shift of the cw photoluminescence band is attributed to an enhancement of an additional broad band, which is caused by deep states, appearing in the low-energy side of the band arising from the transition between band tail electrons and holes. A decrease in field effect mobility in TFTs with an a-SiNx:H gate-insulator was observed when the nitrogen content of the a-SiNx:H increases, and correlated well with the low-energy shift of the cw photoluminescence band. These results suggest that the deep states in a-Si:H near the interface tend to increase with the nitrogen content of the a-SiNx:H. The origin of the states is believed to be a lattice strain induced in the a-Si:H layer. INTRODUCTION Recently, in an amorphous superlattice consisting of a-SiNx:H and a-Si:H layers, Roxlo et al. found that some defects are induced near the a-SiNx :H/a-Si:H interface [i]. Such defects may have an influence on the characteristics of a-Si devices like thin film transistors (TFT) with an a-SiNx:H gate-insulator, as well as superlattice devices. So, characterization of the near-interface properties is important from the viewpoints of both physical interest and device fabrication.
Photoluminescence measurement has been widely used for the nondestructive identification of thin film properties, and is effective in detecting defects. By selecting a suitable excitation energy, it is possible to obtain the photoluminescence spectra of a particular region of a thin film. In the present report, we characterize the near-interface region in an a-SiNx :H/a-Si:H layered structure by photoluminescence measurement, and investigate how the near-interface properties of a-Si:H vary depending on the nitrogen content of a-SiN :H. The results are compared with the field effect mobility in a TFT, and are discussed in terms of concentration of deep states induced in the a-Si:H layer near the interface.
EXPERIMENTAL PROCEDURES Sample preparation The sample structure for photoluminescence measurement is schematically illustrated in Fig.1. A 300 nm thick a-SiNx:H layer and a 100 nm thick aSi:H layer were successively deposited on a ground quartz substrate (600#) by plasma CVD. These layers were deposited in separate chambers to prevent nitrogen contamination of the a-Si:H layer.
Mat. Res. Soc. Symp. Proc. Vol. 70. ' 1986 Materials Res
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