Some electrical properties of amorphous silicon/amorphous silicon nitride interfaces: Top nitride and bottom nitride configurations in MNS and TFT devices

Abstract

Top nitride (TN) and bottom nitride (BN) configurations of the a-SiN1.6:H/a-Si:H interface produced by plasma enhanced chemical vapor deposition (PECVD) have been characterized by using metal/a-SiN1.6:H/a-Si:H (MNS) devices and thin-film transistors (TFT). We observed that: (i) for MNS devices, the resonant frequency activation energy Eac from admittance, saturates at a higher value for the TN (≂0.3 eV) than for the BN (≂0.1 eV) MNS devices and the resonant frequency preexponential factors are found to be, respectively, in the 1012 s−1 and 108 s−1 range, (ii) in strong accumulation, the source-drain current activation energy is nearly similar for TN and BN TFTs (Edc≂0.1 eV), a slightly higher value being found in the latter configuration, (iii) the analysis of the transfer characteristics yields a very similar density of deep interface states (DOS) ≂4–5×1012 cm−2 eV−1 in both configurations, and (iv) the effective field-effect mobility is higher for the BN (μFE≳0.5 cm2/V s) than for the TN (μFE≲0.3 cm2/V s) TFTs. The observed difference in mobilities for both TFTs structures is most likely associated with very high source/drain contact resistances in the TN TFT rather than with the quality of both interfaces. To explain some of these experimental results, the surface- and buried-channel models are proposed for TN and BN TFT configurations, respectively. The buried-channel concept is based on evidence of recycling/intermixing of nitrogen atoms into a-Si:H deposited on a-SiN1.6:H by PECVD. The nitrogen tail would produce a Si-rich a-SiNx:H alloy transition layer, followed by a nitrogen-doped n-type layer. The doped layer corresponding to the buried-channel formation is predicted to be located beyond 30 Å from the BN interface. In contrast, the surface-channel proposed for the TN interface is based upon this interface being considered as atomically sharp. The MNS results (i) are consistent with this model. In the case of TN MNS they are explained by thermionic emission of electrons from the traps to the conduction band edge in a-Si:H and in the case of BN MNS by hopping in the defective a-SiNx:H interlayer. The result (ii) on TFTs may be attributed to a broadening of the linear part of the conduction band tail of a-Si:H in the channel region, due to recycled nitrogen atoms.