Characterization of Localized Density of States in Intrinsic a-Si and poly-Si Films by Transient Voltage Spectroscopy

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INTRINSIC a-Si AND poly-Si FILMS BY TRANSIENT VOLTAGE SPECTROSCOPY G. KAWACHI, M. ISHII, T. TANAKA, AND N. KONISHI Hitachi Research Laboratory, Hitachi Ltd., Hitachi, Ibaraki, Japan

Abstract The localized density of states (LDOS) at interfaces between intrinsic silicon and silicon nitride (Si3N4) films are studied using transient voltage spectroscopy (TVS). In the TVS technique, the transient of the voltage across a MIS-diode after a trap filling voltage pulse is measured using a high-impedance voltage probe. This allows us to make a precise measurement of the LDOS at undoped Si/insulator interfaces. The LDOS in a-Si:H/Si 3N4 systems has a broad peak around the energy of 0.9 eV below the conduction-band edge. A modification of the LDOS at a-Si:H/Si 3N4 interfaces by bias-anneal[ing is clearly observed using this technique. The results are consistent with the defect pool model. The LDOS in laser annealed poly-Si/Si 3N4 systems has a peak centered 0.6eV below the conduction-band edge, which seems to be the Si dangling bond states in the poly-Si films.

Introduction Understanding of the localized density of states (LDOS) at silicon/insulator interfaces is a fundamental issue for improving or predicting the characteristics of Si-based thin-film transistors (TFTs). In general, Si TFTs are fabricated on intrinsic materials. Therefore, precise measurements of LDOS at intrinsic Si/insulator interfaces have especial importance. So far, field-effect conductance [1] and quasi-static capacitance-voltage methods [21 have been used for this purpose. However, deducing LDOS from these experiments are still contradictory, because the results depend strongly on the measurement conditions and the flat-band voltage assumed in the analysis. On the other hand, deep level transient spectroscopy (DLTS) [3] and other related capacitance transient techniques [4-5] have been successfully applied to investigate the LDOS in doped a-Si:H films. These capacitance transient techniques provide information about the energy depth of the traps independent of the flat-band voltage and consequently have a high energy resolution. However, the application of the capacitance transient techniques has been limited to doped materials because of the difficulty in measuring the capacitance in intrinsic materials. This paper describes the application of transient voltage spectroscopy (TVS), a new DLTS technique, for characterizing the LDOS at intrinsic Si/insulator interfaces. With this technique, the modification of the LDOS in a-Si:H/Si 3N4 systems by a bias-anneal treatment is studied. As another interesting application of the TVS, also the LDOS in poly-Si/Si 3N4 interfaces fabricated by the laser annealing technique is shown.

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Mat. Res. Soc. Symp. Proc. Vol. 377 © 1995 Materials Research Society

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t (s) Fig. 2 Schematic diagram of the voltage waveforms in TVS measuments.

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